1PERLFUNC(1)            Perl Programmers Reference Guide            PERLFUNC(1)
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NAME

6       perlfunc - Perl builtin functions
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DESCRIPTION

9       The functions in this section can serve as terms in an expression.
10       They fall into two major categories: list operators and named unary
11       operators.  These differ in their precedence relationship with a
12       following comma.  (See the precedence table in perlop.)  List operators
13       take more than one argument, while unary operators can never take more
14       than one argument.  Thus, a comma terminates the argument of a unary
15       operator, but merely separates the arguments of a list operator.  A
16       unary operator generally provides scalar context to its argument, while
17       a list operator may provide either scalar or list contexts for its
18       arguments.  If it does both, scalar arguments come first and list
19       argument follow, and there can only ever be one such list argument.
20       For instance, "splice" has three scalar arguments followed by a list,
21       whereas "gethostbyname" has four scalar arguments.
22
23       In the syntax descriptions that follow, list operators that expect a
24       list (and provide list context for elements of the list) are shown with
25       LIST as an argument.  Such a list may consist of any combination of
26       scalar arguments or list values; the list values will be included in
27       the list as if each individual element were interpolated at that point
28       in the list, forming a longer single-dimensional list value.  Commas
29       should separate literal elements of the LIST.
30
31       Any function in the list below may be used either with or without
32       parentheses around its arguments.  (The syntax descriptions omit the
33       parentheses.)  If you use parentheses, the simple but occasionally
34       surprising rule is this: It looks like a function, therefore it is a
35       function, and precedence doesn't matter.  Otherwise it's a list
36       operator or unary operator, and precedence does matter.  Whitespace
37       between the function and left parenthesis doesn't count, so sometimes
38       you need to be careful:
39
40           print 1+2+4;      # Prints 7.
41           print(1+2) + 4;   # Prints 3.
42           print (1+2)+4;    # Also prints 3!
43           print +(1+2)+4;   # Prints 7.
44           print ((1+2)+4);  # Prints 7.
45
46       If you run Perl with the "use warnings" pragma, it can warn you about
47       this.  For example, the third line above produces:
48
49           print (...) interpreted as function at - line 1.
50           Useless use of integer addition in void context at - line 1.
51
52       A few functions take no arguments at all, and therefore work as neither
53       unary nor list operators.  These include such functions as "time" and
54       "endpwent".  For example, "time+86_400" always means "time() + 86_400".
55
56       For functions that can be used in either a scalar or list context,
57       nonabortive failure is generally indicated in scalar context by
58       returning the undefined value, and in list context by returning the
59       empty list.
60
61       Remember the following important rule: There is no rule that relates
62       the behavior of an expression in list context to its behavior in scalar
63       context, or vice versa.  It might do two totally different things.
64       Each operator and function decides which sort of value would be most
65       appropriate to return in scalar context.  Some operators return the
66       length of the list that would have been returned in list context.  Some
67       operators return the first value in the list.  Some operators return
68       the last value in the list.  Some operators return a count of
69       successful operations.  In general, they do what you want, unless you
70       want consistency.
71
72       A named array in scalar context is quite different from what would at
73       first glance appear to be a list in scalar context.  You can't get a
74       list like "(1,2,3)" into being in scalar context, because the compiler
75       knows the context at compile time.  It would generate the scalar comma
76       operator there, not the list concatenation version of the comma.  That
77       means it was never a list to start with.
78
79       In general, functions in Perl that serve as wrappers for system calls
80       ("syscalls") of the same name (like chown(2), fork(2), closedir(2),
81       etc.) return true when they succeed and "undef" otherwise, as is
82       usually mentioned in the descriptions below.  This is different from
83       the C interfaces, which return "-1" on failure.  Exceptions to this
84       rule include "wait", "waitpid", and "syscall".  System calls also set
85       the special $! variable on failure.  Other functions do not, except
86       accidentally.
87
88       Extension modules can also hook into the Perl parser to define new
89       kinds of keyword-headed expression.  These may look like functions, but
90       may also look completely different.  The syntax following the keyword
91       is defined entirely by the extension.  If you are an implementor, see
92       "PL_keyword_plugin" in perlapi for the mechanism.  If you are using
93       such a module, see the module's documentation for details of the syntax
94       that it defines.
95
96   Perl Functions by Category
97       Here are Perl's functions (including things that look like functions,
98       like some keywords and named operators) arranged by category.  Some
99       functions appear in more than one place.  Any warnings, including those
100       produced by keywords, are described in perldiag and warnings.
101
102       Functions for SCALARs or strings
103           "chomp", "chop", "chr", "crypt", "fc", "hex", "index", "lc",
104           "lcfirst", "length", "oct", "ord", "pack", "q//", "qq//",
105           "reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst",
106           "y///"
107
108           "fc" is available only if the "fc" feature is enabled or if it is
109           prefixed with "CORE::".  The "fc" feature is enabled automatically
110           with a "use v5.16" (or higher) declaration in the current scope.
111
112       Regular expressions and pattern matching
113           "m//", "pos", "qr//", "quotemeta", "s///", "split", "study"
114
115       Numeric functions
116           "abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand",
117           "sin", "sqrt", "srand"
118
119       Functions for real @ARRAYs
120           "each", "keys", "pop", "push", "shift", "splice", "unshift",
121           "values"
122
123       Functions for list data
124           "grep", "join", "map", "qw//", "reverse", "sort", "unpack"
125
126       Functions for real %HASHes
127           "delete", "each", "exists", "keys", "values"
128
129       Input and output functions
130           "binmode", "close", "closedir", "dbmclose", "dbmopen", "die",
131           "eof", "fileno", "flock", "format", "getc", "print", "printf",
132           "read", "readdir", "readline", "rewinddir", "say", "seek",
133           "seekdir", "select", "syscall", "sysread", "sysseek", "syswrite",
134           "tell", "telldir", "truncate", "warn", "write"
135
136           "say" is available only if the "say" feature is enabled or if it is
137           prefixed with "CORE::".  The "say" feature is enabled automatically
138           with a "use v5.10" (or higher) declaration in the current scope.
139
140       Functions for fixed-length data or records
141           "pack", "read", "syscall", "sysread", "sysseek", "syswrite",
142           "unpack", "vec"
143
144       Functions for filehandles, files, or directories
145           "-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob",
146           "ioctl", "link", "lstat", "mkdir", "open", "opendir", "readlink",
147           "rename", "rmdir", "select", "stat", "symlink", "sysopen", "umask",
148           "unlink", "utime"
149
150       Keywords related to the control flow of your Perl program
151           "break", "caller", "continue", "die", "do", "dump", "eval",
152           "evalbytes", "exit", "__FILE__", "goto", "last", "__LINE__",
153           "next", "__PACKAGE__", "redo", "return", "sub", "__SUB__",
154           "wantarray"
155
156           "break" is available only if you enable the experimental "switch"
157           feature or use the "CORE::" prefix.  The "switch" feature also
158           enables the "default", "given" and "when" statements, which are
159           documented in "Switch Statements" in perlsyn.  The "switch" feature
160           is enabled automatically with a "use v5.10" (or higher) declaration
161           in the current scope.  In Perl v5.14 and earlier, "continue"
162           required the "switch" feature, like the other keywords.
163
164           "evalbytes" is only available with the "evalbytes" feature (see
165           feature) or if prefixed with "CORE::".  "__SUB__" is only available
166           with the "current_sub" feature or if prefixed with "CORE::".  Both
167           the "evalbytes" and "current_sub" features are enabled
168           automatically with a "use v5.16" (or higher) declaration in the
169           current scope.
170
171       Keywords related to scoping
172           "caller", "import", "local", "my", "our", "package", "state", "use"
173
174           "state" is available only if the "state" feature is enabled or if
175           it is prefixed with "CORE::".  The "state" feature is enabled
176           automatically with a "use v5.10" (or higher) declaration in the
177           current scope.
178
179       Miscellaneous functions
180           "defined", "formline", "lock", "prototype", "reset", "scalar",
181           "undef"
182
183       Functions for processes and process groups
184           "alarm", "exec", "fork", "getpgrp", "getppid", "getpriority",
185           "kill", "pipe", "qx//", "readpipe", "setpgrp", "setpriority",
186           "sleep", "system", "times", "wait", "waitpid"
187
188       Keywords related to Perl modules
189           "do", "import", "no", "package", "require", "use"
190
191       Keywords related to classes and object-orientation
192           "bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied",
193           "untie", "use"
194
195       Low-level socket functions
196           "accept", "bind", "connect", "getpeername", "getsockname",
197           "getsockopt", "listen", "recv", "send", "setsockopt", "shutdown",
198           "socket", "socketpair"
199
200       System V interprocess communication functions
201           "msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget",
202           "semop", "shmctl", "shmget", "shmread", "shmwrite"
203
204       Fetching user and group info
205           "endgrent", "endhostent", "endnetent", "endpwent", "getgrent",
206           "getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam",
207           "getpwuid", "setgrent", "setpwent"
208
209       Fetching network info
210           "endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
211           "gethostent", "getnetbyaddr", "getnetbyname", "getnetent",
212           "getprotobyname", "getprotobynumber", "getprotoent",
213           "getservbyname", "getservbyport", "getservent", "sethostent",
214           "setnetent", "setprotoent", "setservent"
215
216       Time-related functions
217           "gmtime", "localtime", "time", "times"
218
219       Non-function keywords
220           "and", "AUTOLOAD", "BEGIN", "CHECK", "cmp", "CORE", "__DATA__",
221           "default", "DESTROY", "else", "elseif", "elsif", "END", "__END__",
222           "eq", "for", "foreach", "ge", "given", "gt", "if", "INIT", "le",
223           "lt", "ne", "not", "or", "UNITCHECK", "unless", "until", "when",
224           "while", "x", "xor"
225
226   Portability
227       Perl was born in Unix and can therefore access all common Unix system
228       calls.  In non-Unix environments, the functionality of some Unix system
229       calls may not be available or details of the available functionality
230       may differ slightly.  The Perl functions affected by this are:
231
232       "-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose",
233       "dbmopen", "dump", "endgrent", "endhostent", "endnetent",
234       "endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock",
235       "fork", "getgrent", "getgrgid", "gethostbyname", "gethostent",
236       "getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid",
237       "getpgrp", "getpriority", "getprotobynumber", "getprotoent",
238       "getpwent", "getpwnam", "getpwuid", "getservbyport", "getservent",
239       "getsockopt", "glob", "ioctl", "kill", "link", "lstat", "msgctl",
240       "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename",
241       "select", "semctl", "semget", "semop", "setgrent", "sethostent",
242       "setnetent", "setpgrp", "setpriority", "setprotoent", "setpwent",
243       "setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
244       "socket", "socketpair", "stat", "symlink", "syscall", "sysopen",
245       "system", "times", "truncate", "umask", "unlink", "utime", "wait",
246       "waitpid"
247
248       For more information about the portability of these functions, see
249       perlport and other available platform-specific documentation.
250
251   Alphabetical Listing of Perl Functions
252       -X FILEHANDLE
253       -X EXPR
254       -X DIRHANDLE
255       -X  A file test, where X is one of the letters listed below.  This
256           unary operator takes one argument, either a filename, a filehandle,
257           or a dirhandle, and tests the associated file to see if something
258           is true about it.  If the argument is omitted, tests $_, except for
259           "-t", which tests STDIN.  Unless otherwise documented, it returns 1
260           for true and '' for false.  If the file doesn't exist or can't be
261           examined, it returns "undef" and sets $! (errno).  With the
262           exception of the "-l" test they all follow symbolic links because
263           they use "stat()" and not "lstat()" (so dangling symlinks can't be
264           examined and will therefore report failure).
265
266           Despite the funny names, precedence is the same as any other named
267           unary operator.  The operator may be any of:
268
269               -r  File is readable by effective uid/gid.
270               -w  File is writable by effective uid/gid.
271               -x  File is executable by effective uid/gid.
272               -o  File is owned by effective uid.
273
274               -R  File is readable by real uid/gid.
275               -W  File is writable by real uid/gid.
276               -X  File is executable by real uid/gid.
277               -O  File is owned by real uid.
278
279               -e  File exists.
280               -z  File has zero size (is empty).
281               -s  File has nonzero size (returns size in bytes).
282
283               -f  File is a plain file.
284               -d  File is a directory.
285               -l  File is a symbolic link (false if symlinks aren't
286                   supported by the file system).
287               -p  File is a named pipe (FIFO), or Filehandle is a pipe.
288               -S  File is a socket.
289               -b  File is a block special file.
290               -c  File is a character special file.
291               -t  Filehandle is opened to a tty.
292
293               -u  File has setuid bit set.
294               -g  File has setgid bit set.
295               -k  File has sticky bit set.
296
297               -T  File is an ASCII or UTF-8 text file (heuristic guess).
298               -B  File is a "binary" file (opposite of -T).
299
300               -M  Script start time minus file modification time, in days.
301               -A  Same for access time.
302               -C  Same for inode change time (Unix, may differ for other
303                   platforms)
304
305           Example:
306
307               while (<>) {
308                   chomp;
309                   next unless -f $_;  # ignore specials
310                   #...
311               }
312
313           Note that "-s/a/b/" does not do a negated substitution.  Saying
314           "-exp($foo)" still works as expected, however: only single letters
315           following a minus are interpreted as file tests.
316
317           These operators are exempt from the "looks like a function rule"
318           described above.  That is, an opening parenthesis after the
319           operator does not affect how much of the following code constitutes
320           the argument.  Put the opening parentheses before the operator to
321           separate it from code that follows (this applies only to operators
322           with higher precedence than unary operators, of course):
323
324               -s($file) + 1024   # probably wrong; same as -s($file + 1024)
325               (-s $file) + 1024  # correct
326
327           The interpretation of the file permission operators "-r", "-R",
328           "-w", "-W", "-x", and "-X" is by default based solely on the mode
329           of the file and the uids and gids of the user.  There may be other
330           reasons you can't actually read, write, or execute the file: for
331           example network filesystem access controls, ACLs (access control
332           lists), read-only filesystems, and unrecognized executable formats.
333           Note that the use of these six specific operators to verify if some
334           operation is possible is usually a mistake, because it may be open
335           to race conditions.
336
337           Also note that, for the superuser on the local filesystems, the
338           "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X"
339           return 1 if any execute bit is set in the mode.  Scripts run by the
340           superuser may thus need to do a "stat" to determine the actual mode
341           of the file, or temporarily set their effective uid to something
342           else.
343
344           If you are using ACLs, there is a pragma called "filetest" that may
345           produce more accurate results than the bare "stat" mode bits.  When
346           under "use filetest 'access'", the above-mentioned filetests test
347           whether the permission can(not) be granted using the access(2)
348           family of system calls.  Also note that the "-x" and "-X" tests may
349           under this pragma return true even if there are no execute
350           permission bits set (nor any extra execute permission ACLs).  This
351           strangeness is due to the underlying system calls' definitions.
352           Note also that, due to the implementation of "use filetest
353           'access'", the "_" special filehandle won't cache the results of
354           the file tests when this pragma is in effect.  Read the
355           documentation for the "filetest" pragma for more information.
356
357           The "-T" and "-B" tests work as follows.  The first block or so of
358           the file is examined to see if it is valid UTF-8 that includes non-
359           ASCII characters.  If so, it's a "-T" file.  Otherwise, that same
360           portion of the file is examined for odd characters such as strange
361           control codes or characters with the high bit set.  If more than a
362           third of the characters are strange, it's a "-B" file; otherwise
363           it's a "-T" file.  Also, any file containing a zero byte in the
364           examined portion is considered a binary file.  (If executed within
365           the scope of a use locale which includes "LC_CTYPE", odd characters
366           are anything that isn't a printable nor space in the current
367           locale.)  If "-T" or "-B" is used on a filehandle, the current IO
368           buffer is examined rather than the first block.  Both "-T" and "-B"
369           return true on an empty file, or a file at EOF when testing a
370           filehandle.  Because you have to read a file to do the "-T" test,
371           on most occasions you want to use a "-f" against the file first, as
372           in "next unless -f $file && -T $file".
373
374           If any of the file tests (or either the "stat" or "lstat" operator)
375           is given the special filehandle consisting of a solitary underline,
376           then the stat structure of the previous file test (or "stat"
377           operator) is used, saving a system call.  (This doesn't work with
378           "-t", and you need to remember that "lstat" and "-l" leave values
379           in the stat structure for the symbolic link, not the real file.)
380           (Also, if the stat buffer was filled by an "lstat" call, "-T" and
381           "-B" will reset it with the results of "stat _").  Example:
382
383               print "Can do.\n" if -r $a || -w _ || -x _;
384
385               stat($filename);
386               print "Readable\n" if -r _;
387               print "Writable\n" if -w _;
388               print "Executable\n" if -x _;
389               print "Setuid\n" if -u _;
390               print "Setgid\n" if -g _;
391               print "Sticky\n" if -k _;
392               print "Text\n" if -T _;
393               print "Binary\n" if -B _;
394
395           As of Perl 5.10.0, as a form of purely syntactic sugar, you can
396           stack file test operators, in a way that "-f -w -x $file" is
397           equivalent to "-x $file && -w _ && -f _".  (This is only fancy
398           syntax: if you use the return value of "-f $file" as an argument to
399           another filetest operator, no special magic will happen.)
400
401           Portability issues: "-X" in perlport.
402
403           To avoid confusing would-be users of your code with mysterious
404           syntax errors, put something like this at the top of your script:
405
406               use 5.010;  # so filetest ops can stack
407
408       abs VALUE
409       abs Returns the absolute value of its argument.  If VALUE is omitted,
410           uses $_.
411
412       accept NEWSOCKET,GENERICSOCKET
413           Accepts an incoming socket connect, just as accept(2) does.
414           Returns the packed address if it succeeded, false otherwise.  See
415           the example in "Sockets: Client/Server Communication" in perlipc.
416
417           On systems that support a close-on-exec flag on files, the flag
418           will be set for the newly opened file descriptor, as determined by
419           the value of $^F.  See "$^F" in perlvar.
420
421       alarm SECONDS
422       alarm
423           Arranges to have a SIGALRM delivered to this process after the
424           specified number of wallclock seconds has elapsed.  If SECONDS is
425           not specified, the value stored in $_ is used.  (On some machines,
426           unfortunately, the elapsed time may be up to one second less or
427           more than you specified because of how seconds are counted, and
428           process scheduling may delay the delivery of the signal even
429           further.)
430
431           Only one timer may be counting at once.  Each call disables the
432           previous timer, and an argument of 0 may be supplied to cancel the
433           previous timer without starting a new one.  The returned value is
434           the amount of time remaining on the previous timer.
435
436           For delays of finer granularity than one second, the Time::HiRes
437           module (from CPAN, and starting from Perl 5.8 part of the standard
438           distribution) provides "ualarm".  You may also use Perl's four-
439           argument version of "select" leaving the first three arguments
440           undefined, or you might be able to use the "syscall" interface to
441           access setitimer(2) if your system supports it.  See perlfaq8 for
442           details.
443
444           It is usually a mistake to intermix "alarm" and "sleep" calls,
445           because "sleep" may be internally implemented on your system with
446           "alarm".
447
448           If you want to use "alarm" to time out a system call you need to
449           use an "eval"/"die" pair.  You can't rely on the alarm causing the
450           system call to fail with $! set to "EINTR" because Perl sets up
451           signal handlers to restart system calls on some systems.  Using
452           "eval"/"die" always works, modulo the caveats given in "Signals" in
453           perlipc.
454
455               eval {
456                   local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
457                   alarm $timeout;
458                   my $nread = sysread $socket, $buffer, $size;
459                   alarm 0;
460               };
461               if ($@) {
462                   die unless $@ eq "alarm\n";   # propagate unexpected errors
463                   # timed out
464               }
465               else {
466                   # didn't
467               }
468
469           For more information see perlipc.
470
471           Portability issues: "alarm" in perlport.
472
473       atan2 Y,X
474           Returns the arctangent of Y/X in the range -PI to PI.
475
476           For the tangent operation, you may use the "Math::Trig::tan"
477           function, or use the familiar relation:
478
479               sub tan { sin($_[0]) / cos($_[0])  }
480
481           The return value for "atan2(0,0)" is implementation-defined;
482           consult your atan2(3) manpage for more information.
483
484           Portability issues: "atan2" in perlport.
485
486       bind SOCKET,NAME
487           Binds a network address to a socket, just as bind(2) does.  Returns
488           true if it succeeded, false otherwise.  NAME should be a packed
489           address of the appropriate type for the socket.  See the examples
490           in "Sockets: Client/Server Communication" in perlipc.
491
492       binmode FILEHANDLE, LAYER
493       binmode FILEHANDLE
494           Arranges for FILEHANDLE to be read or written in "binary" or "text"
495           mode on systems where the run-time libraries distinguish between
496           binary and text files.  If FILEHANDLE is an expression, the value
497           is taken as the name of the filehandle.  Returns true on success,
498           otherwise it returns "undef" and sets $! (errno).
499
500           On some systems (in general, DOS- and Windows-based systems)
501           "binmode" is necessary when you're not working with a text file.
502           For the sake of portability it is a good idea always to use it when
503           appropriate, and never to use it when it isn't appropriate.  Also,
504           people can set their I/O to be by default UTF8-encoded Unicode, not
505           bytes.
506
507           In other words: regardless of platform, use "binmode" on binary
508           data, like images, for example.
509
510           If LAYER is present it is a single string, but may contain multiple
511           directives.  The directives alter the behaviour of the filehandle.
512           When LAYER is present, using binmode on a text file makes sense.
513
514           If LAYER is omitted or specified as ":raw" the filehandle is made
515           suitable for passing binary data.  This includes turning off
516           possible CRLF translation and marking it as bytes (as opposed to
517           Unicode characters).  Note that, despite what may be implied in
518           "Programming Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is
519           not simply the inverse of ":crlf".  Other layers that would affect
520           the binary nature of the stream are also disabled.  See PerlIO, and
521           the discussion about the PERLIO environment variable in perlrun.
522
523           The ":bytes", ":crlf", ":utf8", and any other directives of the
524           form ":...", are called I/O layers.  The open pragma can be used to
525           establish default I/O layers.
526
527           The LAYER parameter of the "binmode" function is described as
528           "DISCIPLINE" in "Programming Perl, 3rd Edition".  However, since
529           the publishing of this book, by many known as "Camel III", the
530           consensus of the naming of this functionality has moved from
531           "discipline" to "layer".  All documentation of this version of Perl
532           therefore refers to "layers" rather than to "disciplines".  Now
533           back to the regularly scheduled documentation...
534
535           To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(UTF-8)".
536           ":utf8" just marks the data as UTF-8 without further checking,
537           while ":encoding(UTF-8)" checks the data for actually being valid
538           UTF-8.  More details can be found in PerlIO::encoding.
539
540           In general, "binmode" should be called after "open" but before any
541           I/O is done on the filehandle.  Calling "binmode" normally flushes
542           any pending buffered output data (and perhaps pending input data)
543           on the handle.  An exception to this is the ":encoding" layer that
544           changes the default character encoding of the handle.  The
545           ":encoding" layer sometimes needs to be called in mid-stream, and
546           it doesn't flush the stream.  ":encoding" also implicitly pushes on
547           top of itself the ":utf8" layer because internally Perl operates on
548           UTF8-encoded Unicode characters.
549
550           The operating system, device drivers, C libraries, and Perl run-
551           time system all conspire to let the programmer treat a single
552           character ("\n") as the line terminator, irrespective of external
553           representation.  On many operating systems, the native text file
554           representation matches the internal representation, but on some
555           platforms the external representation of "\n" is made up of more
556           than one character.
557
558           All variants of Unix, Mac OS (old and new), and Stream_LF files on
559           VMS use a single character to end each line in the external
560           representation of text (even though that single character is
561           CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE
562           FEED on Unix and most VMS files).  In other systems like OS/2, DOS,
563           and the various flavors of MS-Windows, your program sees a "\n" as
564           a simple "\cJ", but what's stored in text files are the two
565           characters "\cM\cJ".  That means that if you don't use "binmode" on
566           these systems, "\cM\cJ" sequences on disk will be converted to "\n"
567           on input, and any "\n" in your program will be converted back to
568           "\cM\cJ" on output.  This is what you want for text files, but it
569           can be disastrous for binary files.
570
571           Another consequence of using "binmode" (on some systems) is that
572           special end-of-file markers will be seen as part of the data
573           stream.  For systems from the Microsoft family this means that, if
574           your binary data contain "\cZ", the I/O subsystem will regard it as
575           the end of the file, unless you use "binmode".
576
577           "binmode" is important not only for "readline" and "print"
578           operations, but also when using "read", "seek", "sysread",
579           "syswrite" and "tell" (see perlport for more details).  See the $/
580           and "$\" variables in perlvar for how to manually set your input
581           and output line-termination sequences.
582
583           Portability issues: "binmode" in perlport.
584
585       bless REF,CLASSNAME
586       bless REF
587           This function tells the thingy referenced by REF that it is now an
588           object in the CLASSNAME package.  If CLASSNAME is an empty string,
589           it is interpreted as referring to the "main" package.  If CLASSNAME
590           is omitted, the current package is used.  Because a "bless" is
591           often the last thing in a constructor, it returns the reference for
592           convenience.  Always use the two-argument version if a derived
593           class might inherit the method doing the blessing.  See perlobj for
594           more about the blessing (and blessings) of objects.
595
596           Consider always blessing objects in CLASSNAMEs that are mixed case.
597           Namespaces with all lowercase names are considered reserved for
598           Perl pragmas.  Builtin types have all uppercase names.  To prevent
599           confusion, you may wish to avoid such package names as well.  It is
600           advised to avoid the class name 0, because much code erroneously
601           uses the result of "ref" as a truth value.
602
603           See "Perl Modules" in perlmod.
604
605       break
606           Break out of a "given" block.
607
608           "break" is available only if the "switch" feature is enabled or if
609           it is prefixed with "CORE::". The "switch" feature is enabled
610           automatically with a "use v5.10" (or higher) declaration in the
611           current scope.
612
613       caller EXPR
614       caller
615           Returns the context of the current pure perl subroutine call.  In
616           scalar context, returns the caller's package name if there is a
617           caller (that is, if we're in a subroutine or "eval" or "require")
618           and the undefined value otherwise.  caller never returns XS subs
619           and they are skipped.  The next pure perl sub will appear instead
620           of the XS sub in caller's return values.  In list context, caller
621           returns
622
623                  # 0         1          2
624               my ($package, $filename, $line) = caller;
625
626           Like "__FILE__" and "__LINE__", the filename and line number
627           returned here may be altered by the mechanism described at "Plain
628           Old Comments (Not!)" in perlsyn.
629
630           With EXPR, it returns some extra information that the debugger uses
631           to print a stack trace.  The value of EXPR indicates how many call
632           frames to go back before the current one.
633
634               #  0         1          2      3            4
635            my ($package, $filename, $line, $subroutine, $hasargs,
636
637               #  5          6          7            8       9         10
638               $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
639             = caller($i);
640
641           Here, $subroutine is the function that the caller called (rather
642           than the function containing the caller).  Note that $subroutine
643           may be "(eval)" if the frame is not a subroutine call, but an
644           "eval".  In such a case additional elements $evaltext and
645           $is_require are set: $is_require is true if the frame is created by
646           a "require" or "use" statement, $evaltext contains the text of the
647           "eval EXPR" statement.  In particular, for an "eval BLOCK"
648           statement, $subroutine is "(eval)", but $evaltext is undefined.
649           (Note also that each "use" statement creates a "require" frame
650           inside an "eval EXPR" frame.)  $subroutine may also be "(unknown)"
651           if this particular subroutine happens to have been deleted from the
652           symbol table.  $hasargs is true if a new instance of @_ was set up
653           for the frame.  $hints and $bitmask contain pragmatic hints that
654           the caller was compiled with.  $hints corresponds to $^H, and
655           $bitmask corresponds to "${^WARNING_BITS}".  The $hints and
656           $bitmask values are subject to change between versions of Perl, and
657           are not meant for external use.
658
659           $hinthash is a reference to a hash containing the value of "%^H"
660           when the caller was compiled, or "undef" if "%^H" was empty.  Do
661           not modify the values of this hash, as they are the actual values
662           stored in the optree.
663
664           Note that the only types of call frames that are visible are
665           subroutine calls and "eval". Other forms of context, such as
666           "while" or "foreach" loops or "try" blocks are not considered
667           interesting to "caller", as they do not alter the behaviour of the
668           "return" expression.
669
670           Furthermore, when called from within the DB package in list
671           context, and with an argument, caller returns more detailed
672           information: it sets the list variable @DB::args to be the
673           arguments with which the subroutine was invoked.
674
675           Be aware that the optimizer might have optimized call frames away
676           before "caller" had a chance to get the information.  That means
677           that caller(N) might not return information about the call frame
678           you expect it to, for "N > 1".  In particular, @DB::args might have
679           information from the previous time "caller" was called.
680
681           Be aware that setting @DB::args is best effort, intended for
682           debugging or generating backtraces, and should not be relied upon.
683           In particular, as @_ contains aliases to the caller's arguments,
684           Perl does not take a copy of @_, so @DB::args will contain
685           modifications the subroutine makes to @_ or its contents, not the
686           original values at call time.  @DB::args, like @_, does not hold
687           explicit references to its elements, so under certain cases its
688           elements may have become freed and reallocated for other variables
689           or temporary values.  Finally, a side effect of the current
690           implementation is that the effects of "shift @_" can normally be
691           undone (but not "pop @_" or other splicing, and not if a reference
692           to @_ has been taken, and subject to the caveat about reallocated
693           elements), so @DB::args is actually a hybrid of the current state
694           and initial state of @_.  Buyer beware.
695
696       chdir EXPR
697       chdir FILEHANDLE
698       chdir DIRHANDLE
699       chdir
700           Changes the working directory to EXPR, if possible.  If EXPR is
701           omitted, changes to the directory specified by $ENV{HOME}, if set;
702           if not, changes to the directory specified by $ENV{LOGDIR}.  (Under
703           VMS, the variable $ENV{'SYS$LOGIN'} is also checked, and used if it
704           is set.)  If neither is set, "chdir" does nothing and fails.  It
705           returns true on success, false otherwise.  See the example under
706           "die".
707
708           On systems that support fchdir(2), you may pass a filehandle or
709           directory handle as the argument.  On systems that don't support
710           fchdir(2), passing handles raises an exception.
711
712       chmod LIST
713           Changes the permissions of a list of files.  The first element of
714           the list must be the numeric mode, which should probably be an
715           octal number, and which definitely should not be a string of octal
716           digits: 0644 is okay, but "0644" is not.  Returns the number of
717           files successfully changed.  See also "oct" if all you have is a
718           string.
719
720               my $cnt = chmod 0755, "foo", "bar";
721               chmod 0755, @executables;
722               my $mode = "0644"; chmod $mode, "foo";      # !!! sets mode to
723                                                           # --w----r-T
724               my $mode = "0644"; chmod oct($mode), "foo"; # this is better
725               my $mode = 0644;   chmod $mode, "foo";      # this is best
726
727           On systems that support fchmod(2), you may pass filehandles among
728           the files.  On systems that don't support fchmod(2), passing
729           filehandles raises an exception.  Filehandles must be passed as
730           globs or glob references to be recognized; barewords are considered
731           filenames.
732
733               open(my $fh, "<", "foo");
734               my $perm = (stat $fh)[2] & 07777;
735               chmod($perm | 0600, $fh);
736
737           You can also import the symbolic "S_I*" constants from the "Fcntl"
738           module:
739
740               use Fcntl qw( :mode );
741               chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
742               # Identical to the chmod 0755 of the example above.
743
744           Portability issues: "chmod" in perlport.
745
746       chomp VARIABLE
747       chomp( LIST )
748       chomp
749           This safer version of "chop" removes any trailing string that
750           corresponds to the current value of $/ (also known as
751           $INPUT_RECORD_SEPARATOR in the "English" module).  It returns the
752           total number of characters removed from all its arguments.  It's
753           often used to remove the newline from the end of an input record
754           when you're worried that the final record may be missing its
755           newline.  When in paragraph mode ("$/ = ''"), it removes all
756           trailing newlines from the string.  When in slurp mode ("$/ =
757           undef") or fixed-length record mode ($/ is a reference to an
758           integer or the like; see perlvar), "chomp" won't remove anything.
759           If VARIABLE is omitted, it chomps $_.  Example:
760
761               while (<>) {
762                   chomp;  # avoid \n on last field
763                   my @array = split(/:/);
764                   # ...
765               }
766
767           If VARIABLE is a hash, it chomps the hash's values, but not its
768           keys, resetting the "each" iterator in the process.
769
770           You can actually chomp anything that's an lvalue, including an
771           assignment:
772
773               chomp(my $cwd = `pwd`);
774               chomp(my $answer = <STDIN>);
775
776           If you chomp a list, each element is chomped, and the total number
777           of characters removed is returned.
778
779           Note that parentheses are necessary when you're chomping anything
780           that is not a simple variable.  This is because "chomp $cwd =
781           `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as
782           "chomp( $cwd = `pwd` )" which you might expect.  Similarly, "chomp
783           $a, $b" is interpreted as "chomp($a), $b" rather than as "chomp($a,
784           $b)".
785
786       chop VARIABLE
787       chop( LIST )
788       chop
789           Chops off the last character of a string and returns the character
790           chopped.  It is much more efficient than "s/.$//s" because it
791           neither scans nor copies the string.  If VARIABLE is omitted, chops
792           $_.  If VARIABLE is a hash, it chops the hash's values, but not its
793           keys, resetting the "each" iterator in the process.
794
795           You can actually chop anything that's an lvalue, including an
796           assignment.
797
798           If you chop a list, each element is chopped.  Only the value of the
799           last "chop" is returned.
800
801           Note that "chop" returns the last character.  To return all but the
802           last character, use "substr($string, 0, -1)".
803
804           See also "chomp".
805
806       chown LIST
807           Changes the owner (and group) of a list of files.  The first two
808           elements of the list must be the numeric uid and gid, in that
809           order.  A value of -1 in either position is interpreted by most
810           systems to leave that value unchanged.  Returns the number of files
811           successfully changed.
812
813               my $cnt = chown $uid, $gid, 'foo', 'bar';
814               chown $uid, $gid, @filenames;
815
816           On systems that support fchown(2), you may pass filehandles among
817           the files.  On systems that don't support fchown(2), passing
818           filehandles raises an exception.  Filehandles must be passed as
819           globs or glob references to be recognized; barewords are considered
820           filenames.
821
822           Here's an example that looks up nonnumeric uids in the passwd file:
823
824               print "User: ";
825               chomp(my $user = <STDIN>);
826               print "Files: ";
827               chomp(my $pattern = <STDIN>);
828
829               my ($login,$pass,$uid,$gid) = getpwnam($user)
830                   or die "$user not in passwd file";
831
832               my @ary = glob($pattern);  # expand filenames
833               chown $uid, $gid, @ary;
834
835           On most systems, you are not allowed to change the ownership of the
836           file unless you're the superuser, although you should be able to
837           change the group to any of your secondary groups.  On insecure
838           systems, these restrictions may be relaxed, but this is not a
839           portable assumption.  On POSIX systems, you can detect this
840           condition this way:
841
842               use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
843               my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);
844
845           Portability issues: "chown" in perlport.
846
847       chr NUMBER
848       chr Returns the character represented by that NUMBER in the character
849           set.  For example, "chr(65)" is "A" in either ASCII or Unicode, and
850           chr(0x263a) is a Unicode smiley face.
851
852           Negative values give the Unicode replacement character
853           (chr(0xfffd)), except under the bytes pragma, where the low eight
854           bits of the value (truncated to an integer) are used.
855
856           If NUMBER is omitted, uses $_.
857
858           For the reverse, use "ord".
859
860           Note that characters from 128 to 255 (inclusive) are by default
861           internally not encoded as UTF-8 for backward compatibility reasons.
862
863           See perlunicode for more about Unicode.
864
865       chroot FILENAME
866       chroot
867           This function works like the system call by the same name: it makes
868           the named directory the new root directory for all further
869           pathnames that begin with a "/" by your process and all its
870           children.  (It doesn't change your current working directory, which
871           is unaffected.)  For security reasons, this call is restricted to
872           the superuser.  If FILENAME is omitted, does a "chroot" to $_.
873
874           NOTE:  It is mandatory for security to "chdir("/")" ("chdir" to the
875           root directory) immediately after a "chroot", otherwise the current
876           working directory may be outside of the new root.
877
878           Portability issues: "chroot" in perlport.
879
880       close FILEHANDLE
881       close
882           Closes the file or pipe associated with the filehandle, flushes the
883           IO buffers, and closes the system file descriptor.  Returns true if
884           those operations succeed and if no error was reported by any PerlIO
885           layer.  Closes the currently selected filehandle if the argument is
886           omitted.
887
888           You don't have to close FILEHANDLE if you are immediately going to
889           do another "open" on it, because "open" closes it for you.  (See
890           "open".) However, an explicit "close" on an input file resets the
891           line counter ($.), while the implicit close done by "open" does
892           not.
893
894           If the filehandle came from a piped open, "close" returns false if
895           one of the other syscalls involved fails or if its program exits
896           with non-zero status.  If the only problem was that the program
897           exited non-zero, $! will be set to 0.  Closing a pipe also waits
898           for the process executing on the pipe to exit--in case you wish to
899           look at the output of the pipe afterwards--and implicitly puts the
900           exit status value of that command into $? and
901           "${^CHILD_ERROR_NATIVE}".
902
903           If there are multiple threads running, "close" on a filehandle from
904           a piped open returns true without waiting for the child process to
905           terminate, if the filehandle is still open in another thread.
906
907           Closing the read end of a pipe before the process writing to it at
908           the other end is done writing results in the writer receiving a
909           SIGPIPE.  If the other end can't handle that, be sure to read all
910           the data before closing the pipe.
911
912           Example:
913
914               open(OUTPUT, '|sort >foo')  # pipe to sort
915                   or die "Can't start sort: $!";
916               #...                        # print stuff to output
917               close OUTPUT                # wait for sort to finish
918                   or warn $! ? "Error closing sort pipe: $!"
919                              : "Exit status $? from sort";
920               open(INPUT, 'foo')          # get sort's results
921                   or die "Can't open 'foo' for input: $!";
922
923           FILEHANDLE may be an expression whose value can be used as an
924           indirect filehandle, usually the real filehandle name or an
925           autovivified handle.
926
927       closedir DIRHANDLE
928           Closes a directory opened by "opendir" and returns the success of
929           that system call.
930
931       connect SOCKET,NAME
932           Attempts to connect to a remote socket, just like connect(2).
933           Returns true if it succeeded, false otherwise.  NAME should be a
934           packed address of the appropriate type for the socket.  See the
935           examples in "Sockets: Client/Server Communication" in perlipc.
936
937       continue BLOCK
938       continue
939           When followed by a BLOCK, "continue" is actually a flow control
940           statement rather than a function.  If there is a "continue" BLOCK
941           attached to a BLOCK (typically in a "while" or "foreach"), it is
942           always executed just before the conditional is about to be
943           evaluated again, just like the third part of a "for" loop in C.
944           Thus it can be used to increment a loop variable, even when the
945           loop has been continued via the "next" statement (which is similar
946           to the C "continue" statement).
947
948           "last", "next", or "redo" may appear within a "continue" block;
949           "last" and "redo" behave as if they had been executed within the
950           main block.  So will "next", but since it will execute a "continue"
951           block, it may be more entertaining.
952
953               while (EXPR) {
954                   ### redo always comes here
955                   do_something;
956               } continue {
957                   ### next always comes here
958                   do_something_else;
959                   # then back the top to re-check EXPR
960               }
961               ### last always comes here
962
963           Omitting the "continue" section is equivalent to using an empty
964           one, logically enough, so "next" goes directly back to check the
965           condition at the top of the loop.
966
967           When there is no BLOCK, "continue" is a function that falls through
968           the current "when" or "default" block instead of iterating a
969           dynamically enclosing "foreach" or exiting a lexically enclosing
970           "given".  In Perl 5.14 and earlier, this form of "continue" was
971           only available when the "switch" feature was enabled.  See feature
972           and "Switch Statements" in perlsyn for more information.
973
974       cos EXPR
975       cos Returns the cosine of EXPR (expressed in radians).  If EXPR is
976           omitted, takes the cosine of $_.
977
978           For the inverse cosine operation, you may use the
979           "Math::Trig::acos" function, or use this relation:
980
981               sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
982
983       crypt PLAINTEXT,SALT
984           Creates a digest string exactly like the crypt(3) function in the C
985           library (assuming that you actually have a version there that has
986           not been extirpated as a potential munition).
987
988           "crypt" is a one-way hash function.  The PLAINTEXT and SALT are
989           turned into a short string, called a digest, which is returned.
990           The same PLAINTEXT and SALT will always return the same string, but
991           there is no (known) way to get the original PLAINTEXT from the
992           hash.  Small changes in the PLAINTEXT or SALT will result in large
993           changes in the digest.
994
995           There is no decrypt function.  This function isn't all that useful
996           for cryptography (for that, look for Crypt modules on your nearby
997           CPAN mirror) and the name "crypt" is a bit of a misnomer.  Instead
998           it is primarily used to check if two pieces of text are the same
999           without having to transmit or store the text itself.  An example is
1000           checking if a correct password is given.  The digest of the
1001           password is stored, not the password itself.  The user types in a
1002           password that is "crypt"'d with the same salt as the stored digest.
1003           If the two digests match, the password is correct.
1004
1005           When verifying an existing digest string you should use the digest
1006           as the salt (like "crypt($plain, $digest) eq $digest").  The SALT
1007           used to create the digest is visible as part of the digest.  This
1008           ensures "crypt" will hash the new string with the same salt as the
1009           digest.  This allows your code to work with the standard "crypt"
1010           and with more exotic implementations.  In other words, assume
1011           nothing about the returned string itself nor about how many bytes
1012           of SALT may matter.
1013
1014           Traditionally the result is a string of 13 bytes: two first bytes
1015           of the salt, followed by 11 bytes from the set "[./0-9A-Za-z]", and
1016           only the first eight bytes of PLAINTEXT mattered.  But alternative
1017           hashing schemes (like MD5), higher level security schemes (like
1018           C2), and implementations on non-Unix platforms may produce
1019           different strings.
1020
1021           When choosing a new salt create a random two character string whose
1022           characters come from the set "[./0-9A-Za-z]" (like "join '', ('.',
1023           '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").  This set of
1024           characters is just a recommendation; the characters allowed in the
1025           salt depend solely on your system's crypt library, and Perl can't
1026           restrict what salts "crypt" accepts.
1027
1028           Here's an example that makes sure that whoever runs this program
1029           knows their password:
1030
1031               my $pwd = (getpwuid($<))[1];
1032
1033               system "stty -echo";
1034               print "Password: ";
1035               chomp(my $word = <STDIN>);
1036               print "\n";
1037               system "stty echo";
1038
1039               if (crypt($word, $pwd) ne $pwd) {
1040                   die "Sorry...\n";
1041               } else {
1042                   print "ok\n";
1043               }
1044
1045           Of course, typing in your own password to whoever asks you for it
1046           is unwise.
1047
1048           The "crypt" function is unsuitable for hashing large quantities of
1049           data, not least of all because you can't get the information back.
1050           Look at the Digest module for more robust algorithms.
1051
1052           If using "crypt" on a Unicode string (which potentially has
1053           characters with codepoints above 255), Perl tries to make sense of
1054           the situation by trying to downgrade (a copy of) the string back to
1055           an eight-bit byte string before calling "crypt" (on that copy).  If
1056           that works, good.  If not, "crypt" dies with "Wide character in
1057           crypt".
1058
1059           Portability issues: "crypt" in perlport.
1060
1061       dbmclose HASH
1062           [This function has been largely superseded by the "untie"
1063           function.]
1064
1065           Breaks the binding between a DBM file and a hash.
1066
1067           Portability issues: "dbmclose" in perlport.
1068
1069       dbmopen HASH,DBNAME,MASK
1070           [This function has been largely superseded by the "tie" function.]
1071
1072           This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file
1073           to a hash.  HASH is the name of the hash.  (Unlike normal "open",
1074           the first argument is not a filehandle, even though it looks like
1075           one).  DBNAME is the name of the database (without the .dir or .pag
1076           extension if any).  If the database does not exist, it is created
1077           with protection specified by MASK (as modified by the "umask").  To
1078           prevent creation of the database if it doesn't exist, you may
1079           specify a MODE of 0, and the function will return a false value if
1080           it can't find an existing database.  If your system supports only
1081           the older DBM functions, you may make only one "dbmopen" call in
1082           your program.  In older versions of Perl, if your system had
1083           neither DBM nor ndbm, calling "dbmopen" produced a fatal error; it
1084           now falls back to sdbm(3).
1085
1086           If you don't have write access to the DBM file, you can only read
1087           hash variables, not set them.  If you want to test whether you can
1088           write, either use file tests or try setting a dummy hash entry
1089           inside an "eval" to trap the error.
1090
1091           Note that functions such as "keys" and "values" may return huge
1092           lists when used on large DBM files.  You may prefer to use the
1093           "each" function to iterate over large DBM files.  Example:
1094
1095               # print out history file offsets
1096               dbmopen(%HIST,'/usr/lib/news/history',0666);
1097               while (($key,$val) = each %HIST) {
1098                   print $key, ' = ', unpack('L',$val), "\n";
1099               }
1100               dbmclose(%HIST);
1101
1102           See also AnyDBM_File for a more general description of the pros and
1103           cons of the various dbm approaches, as well as DB_File for a
1104           particularly rich implementation.
1105
1106           You can control which DBM library you use by loading that library
1107           before you call "dbmopen":
1108
1109               use DB_File;
1110               dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1111                   or die "Can't open netscape history file: $!";
1112
1113           Portability issues: "dbmopen" in perlport.
1114
1115       defined EXPR
1116       defined
1117           Returns a Boolean value telling whether EXPR has a value other than
1118           the undefined value "undef".  If EXPR is not present, $_ is
1119           checked.
1120
1121           Many operations return "undef" to indicate failure, end of file,
1122           system error, uninitialized variable, and other exceptional
1123           conditions.  This function allows you to distinguish "undef" from
1124           other values.  (A simple Boolean test will not distinguish among
1125           "undef", zero, the empty string, and "0", which are all equally
1126           false.)  Note that since "undef" is a valid scalar, its presence
1127           doesn't necessarily indicate an exceptional condition: "pop"
1128           returns "undef" when its argument is an empty array, or when the
1129           element to return happens to be "undef".
1130
1131           You may also use "defined(&func)" to check whether subroutine
1132           "func" has ever been defined.  The return value is unaffected by
1133           any forward declarations of "func".  A subroutine that is not
1134           defined may still be callable: its package may have an "AUTOLOAD"
1135           method that makes it spring into existence the first time that it
1136           is called; see perlsub.
1137
1138           Use of "defined" on aggregates (hashes and arrays) is no longer
1139           supported. It used to report whether memory for that aggregate had
1140           ever been allocated.  You should instead use a simple test for
1141           size:
1142
1143               if (@an_array) { print "has array elements\n" }
1144               if (%a_hash)   { print "has hash members\n"   }
1145
1146           When used on a hash element, it tells you whether the value is
1147           defined, not whether the key exists in the hash.  Use "exists" for
1148           the latter purpose.
1149
1150           Examples:
1151
1152               print if defined $switch{D};
1153               print "$val\n" while defined($val = pop(@ary));
1154               die "Can't readlink $sym: $!"
1155                   unless defined($value = readlink $sym);
1156               sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
1157               $debugging = 0 unless defined $debugging;
1158
1159           Note:  Many folks tend to overuse "defined" and are then surprised
1160           to discover that the number 0 and "" (the zero-length string) are,
1161           in fact, defined values.  For example, if you say
1162
1163               "ab" =~ /a(.*)b/;
1164
1165           The pattern match succeeds and $1 is defined, although it matched
1166           "nothing".  It didn't really fail to match anything.  Rather, it
1167           matched something that happened to be zero characters long.  This
1168           is all very above-board and honest.  When a function returns an
1169           undefined value, it's an admission that it couldn't give you an
1170           honest answer.  So you should use "defined" only when questioning
1171           the integrity of what you're trying to do.  At other times, a
1172           simple comparison to 0 or "" is what you want.
1173
1174           See also "undef", "exists", "ref".
1175
1176       delete EXPR
1177           Given an expression that specifies an element or slice of a hash,
1178           "delete" deletes the specified elements from that hash so that
1179           "exists" on that element no longer returns true.  Setting a hash
1180           element to the undefined value does not remove its key, but
1181           deleting it does; see "exists".
1182
1183           In list context, usually returns the value or values deleted, or
1184           the last such element in scalar context.  The return list's length
1185           corresponds to that of the argument list: deleting non-existent
1186           elements returns the undefined value in their corresponding
1187           positions. When a key/value hash slice is passed to "delete", the
1188           return value is a list of key/value pairs (two elements for each
1189           item deleted from the hash).
1190
1191           "delete" may also be used on arrays and array slices, but its
1192           behavior is less straightforward.  Although "exists" will return
1193           false for deleted entries, deleting array elements never changes
1194           indices of existing values; use "shift" or "splice" for that.
1195           However, if any deleted elements fall at the end of an array, the
1196           array's size shrinks to the position of the highest element that
1197           still tests true for "exists", or to 0 if none do.  In other words,
1198           an array won't have trailing nonexistent elements after a delete.
1199
1200           WARNING: Calling "delete" on array values is strongly discouraged.
1201           The notion of deleting or checking the existence of Perl array
1202           elements is not conceptually coherent, and can lead to surprising
1203           behavior.
1204
1205           Deleting from %ENV modifies the environment.  Deleting from a hash
1206           tied to a DBM file deletes the entry from the DBM file.  Deleting
1207           from a "tied" hash or array may not necessarily return anything; it
1208           depends on the implementation of the "tied" package's DELETE
1209           method, which may do whatever it pleases.
1210
1211           The "delete local EXPR" construct localizes the deletion to the
1212           current block at run time.  Until the block exits, elements locally
1213           deleted temporarily no longer exist.  See "Localized deletion of
1214           elements of composite types" in perlsub.
1215
1216               my %hash = (foo => 11, bar => 22, baz => 33);
1217               my $scalar = delete $hash{foo};         # $scalar is 11
1218               $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1219               my @array  = delete @hash{qw(foo baz)}; # @array  is (undef,33)
1220
1221           The following (inefficiently) deletes all the values of %HASH and
1222           @ARRAY:
1223
1224               foreach my $key (keys %HASH) {
1225                   delete $HASH{$key};
1226               }
1227
1228               foreach my $index (0 .. $#ARRAY) {
1229                   delete $ARRAY[$index];
1230               }
1231
1232           And so do these:
1233
1234               delete @HASH{keys %HASH};
1235
1236               delete @ARRAY[0 .. $#ARRAY];
1237
1238           But both are slower than assigning the empty list or undefining
1239           %HASH or @ARRAY, which is the customary way to empty out an
1240           aggregate:
1241
1242               %HASH = ();     # completely empty %HASH
1243               undef %HASH;    # forget %HASH ever existed
1244
1245               @ARRAY = ();    # completely empty @ARRAY
1246               undef @ARRAY;   # forget @ARRAY ever existed
1247
1248           The EXPR can be arbitrarily complicated provided its final
1249           operation is an element or slice of an aggregate:
1250
1251               delete $ref->[$x][$y]{$key};
1252               delete $ref->[$x][$y]->@{$key1, $key2, @morekeys};
1253
1254               delete $ref->[$x][$y][$index];
1255               delete $ref->[$x][$y]->@[$index1, $index2, @moreindices];
1256
1257       die LIST
1258           "die" raises an exception.  Inside an "eval" the exception is
1259           stuffed into $@ and the "eval" is terminated with the undefined
1260           value.  If the exception is outside of all enclosing "eval"s, then
1261           the uncaught exception is printed to "STDERR" and perl exits with
1262           an exit code indicating failure.  If you need to exit the process
1263           with a specific exit code, see "exit".
1264
1265           Equivalent examples:
1266
1267               die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1268               chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1269
1270           Most of the time, "die" is called with a string to use as the
1271           exception.  You may either give a single non-reference operand to
1272           serve as the exception, or a list of two or more items, which will
1273           be stringified and concatenated to make the exception.
1274
1275           If the string exception does not end in a newline, the current
1276           script line number and input line number (if any) and a newline are
1277           appended to it.  Note that the "input line number" (also known as
1278           "chunk") is subject to whatever notion of "line" happens to be
1279           currently in effect, and is also available as the special variable
1280           $..  See "$/" in perlvar and "$." in perlvar.
1281
1282           Hint: sometimes appending ", stopped" to your message will cause it
1283           to make better sense when the string "at foo line 123" is appended.
1284           Suppose you are running script "canasta".
1285
1286               die "/etc/games is no good";
1287               die "/etc/games is no good, stopped";
1288
1289           produce, respectively
1290
1291               /etc/games is no good at canasta line 123.
1292               /etc/games is no good, stopped at canasta line 123.
1293
1294           If LIST was empty or made an empty string, and $@ already contains
1295           an exception value (typically from a previous "eval"), then that
1296           value is reused after appending "\t...propagated".  This is useful
1297           for propagating exceptions:
1298
1299               eval { ... };
1300               die unless $@ =~ /Expected exception/;
1301
1302           If LIST was empty or made an empty string, and $@ contains an
1303           object reference that has a "PROPAGATE" method, that method will be
1304           called with additional file and line number parameters.  The return
1305           value replaces the value in $@;  i.e., as if "$@ = eval {
1306           $@->PROPAGATE(__FILE__, __LINE__) };" were called.
1307
1308           If LIST was empty or made an empty string, and $@ is also empty,
1309           then the string "Died" is used.
1310
1311           You can also call "die" with a reference argument, and if this is
1312           trapped within an "eval", $@ contains that reference.  This permits
1313           more elaborate exception handling using objects that maintain
1314           arbitrary state about the exception.  Such a scheme is sometimes
1315           preferable to matching particular string values of $@ with regular
1316           expressions.
1317
1318           Because Perl stringifies uncaught exception messages before
1319           display, you'll probably want to overload stringification
1320           operations on exception objects.  See overload for details about
1321           that.  The stringified message should be non-empty, and should end
1322           in a newline, in order to fit in with the treatment of string
1323           exceptions.  Also, because an exception object reference cannot be
1324           stringified without destroying it, Perl doesn't attempt to append
1325           location or other information to a reference exception.  If you
1326           want location information with a complex exception object, you'll
1327           have to arrange to put the location information into the object
1328           yourself.
1329
1330           Because $@ is a global variable, be careful that analyzing an
1331           exception caught by "eval" doesn't replace the reference in the
1332           global variable.  It's easiest to make a local copy of the
1333           reference before any manipulations.  Here's an example:
1334
1335               use Scalar::Util "blessed";
1336
1337               eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1338               if (my $ev_err = $@) {
1339                   if (blessed($ev_err)
1340                       && $ev_err->isa("Some::Module::Exception")) {
1341                       # handle Some::Module::Exception
1342                   }
1343                   else {
1344                       # handle all other possible exceptions
1345                   }
1346               }
1347
1348           If an uncaught exception results in interpreter exit, the exit code
1349           is determined from the values of $! and $? with this pseudocode:
1350
1351               exit $! if $!;              # errno
1352               exit $? >> 8 if $? >> 8;    # child exit status
1353               exit 255;                   # last resort
1354
1355           As with "exit", $? is set prior to unwinding the call stack; any
1356           "DESTROY" or "END" handlers can then alter this value, and thus
1357           Perl's exit code.
1358
1359           The intent is to squeeze as much possible information about the
1360           likely cause into the limited space of the system exit code.
1361           However, as $! is the value of C's "errno", which can be set by any
1362           system call, this means that the value of the exit code used by
1363           "die" can be non-predictable, so should not be relied upon, other
1364           than to be non-zero.
1365
1366           You can arrange for a callback to be run just before the "die" does
1367           its deed, by setting the $SIG{__DIE__} hook.  The associated
1368           handler is called with the exception as an argument, and can change
1369           the exception, if it sees fit, by calling "die" again.  See "%SIG"
1370           in perlvar for details on setting %SIG entries, and "eval" for some
1371           examples.  Although this feature was to be run only right before
1372           your program was to exit, this is not currently so: the
1373           $SIG{__DIE__} hook is currently called even inside "eval"ed
1374           blocks/strings!  If one wants the hook to do nothing in such
1375           situations, put
1376
1377               die @_ if $^S;
1378
1379           as the first line of the handler (see "$^S" in perlvar).  Because
1380           this promotes strange action at a distance, this counterintuitive
1381           behavior may be fixed in a future release.
1382
1383           See also "exit", "warn", and the Carp module.
1384
1385       do BLOCK
1386           Not really a function.  Returns the value of the last command in
1387           the sequence of commands indicated by BLOCK.  When modified by the
1388           "while" or "until" loop modifier, executes the BLOCK once before
1389           testing the loop condition.  (On other statements the loop
1390           modifiers test the conditional first.)
1391
1392           "do BLOCK" does not count as a loop, so the loop control statements
1393           "next", "last", or "redo" cannot be used to leave or restart the
1394           block.  See perlsyn for alternative strategies.
1395
1396       do EXPR
1397           Uses the value of EXPR as a filename and executes the contents of
1398           the file as a Perl script:
1399
1400               # load the exact specified file (./ and ../ special-cased)
1401               do '/foo/stat.pl';
1402               do './stat.pl';
1403               do '../foo/stat.pl';
1404
1405               # search for the named file within @INC
1406               do 'stat.pl';
1407               do 'foo/stat.pl';
1408
1409           "do './stat.pl'" is largely like
1410
1411               eval `cat stat.pl`;
1412
1413           except that it's more concise, runs no external processes, and
1414           keeps track of the current filename for error messages. It also
1415           differs in that code evaluated with "do FILE" cannot see lexicals
1416           in the enclosing scope; "eval STRING" does.  It's the same,
1417           however, in that it does reparse the file every time you call it,
1418           so you probably don't want to do this inside a loop.
1419
1420           Using "do" with a relative path (except for ./ and ../), like
1421
1422               do 'foo/stat.pl';
1423
1424           will search the @INC directories, and update %INC if the file is
1425           found.  See "@INC" in perlvar and "%INC" in perlvar for these
1426           variables. In particular, note that whilst historically @INC
1427           contained '.' (the current directory) making these two cases
1428           equivalent, that is no longer necessarily the case, as '.' is not
1429           included in @INC by default in perl versions 5.26.0 onwards.
1430           Instead, perl will now warn:
1431
1432               do "stat.pl" failed, '.' is no longer in @INC;
1433               did you mean do "./stat.pl"?
1434
1435           If "do" can read the file but cannot compile it, it returns "undef"
1436           and sets an error message in $@.  If "do" cannot read the file, it
1437           returns undef and sets $! to the error.  Always check $@ first, as
1438           compilation could fail in a way that also sets $!.  If the file is
1439           successfully compiled, "do" returns the value of the last
1440           expression evaluated.
1441
1442           Inclusion of library modules is better done with the "use" and
1443           "require" operators, which also do automatic error checking and
1444           raise an exception if there's a problem.
1445
1446           You might like to use "do" to read in a program configuration file.
1447           Manual error checking can be done this way:
1448
1449               # Read in config files: system first, then user.
1450               # Beware of using relative pathnames here.
1451               for $file ("/share/prog/defaults.rc",
1452                          "$ENV{HOME}/.someprogrc")
1453               {
1454                   unless ($return = do $file) {
1455                       warn "couldn't parse $file: $@" if $@;
1456                       warn "couldn't do $file: $!"    unless defined $return;
1457                       warn "couldn't run $file"       unless $return;
1458                   }
1459               }
1460
1461       dump LABEL
1462       dump EXPR
1463       dump
1464           This function causes an immediate core dump.  See also the -u
1465           command-line switch in perlrun, which does the same thing.
1466           Primarily this is so that you can use the undump program (not
1467           supplied) to turn your core dump into an executable binary after
1468           having initialized all your variables at the beginning of the
1469           program.  When the new binary is executed it will begin by
1470           executing a "goto LABEL" (with all the restrictions that "goto"
1471           suffers).  Think of it as a goto with an intervening core dump and
1472           reincarnation.  If "LABEL" is omitted, restarts the program from
1473           the top.  The "dump EXPR" form, available starting in Perl 5.18.0,
1474           allows a name to be computed at run time, being otherwise identical
1475           to "dump LABEL".
1476
1477           WARNING: Any files opened at the time of the dump will not be open
1478           any more when the program is reincarnated, with possible resulting
1479           confusion by Perl.
1480
1481           This function is now largely obsolete, mostly because it's very
1482           hard to convert a core file into an executable.  As of Perl 5.30,
1483           it must be invoked as "CORE::dump()".
1484
1485           Unlike most named operators, this has the same precedence as
1486           assignment.  It is also exempt from the looks-like-a-function rule,
1487           so "dump ("foo")."bar"" will cause "bar" to be part of the argument
1488           to "dump".
1489
1490           Portability issues: "dump" in perlport.
1491
1492       each HASH
1493       each ARRAY
1494           When called on a hash in list context, returns a 2-element list
1495           consisting of the key and value for the next element of a hash.  In
1496           Perl 5.12 and later only, it will also return the index and value
1497           for the next element of an array so that you can iterate over it;
1498           older Perls consider this a syntax error.  When called in scalar
1499           context, returns only the key (not the value) in a hash, or the
1500           index in an array.
1501
1502           Hash entries are returned in an apparently random order.  The
1503           actual random order is specific to a given hash; the exact same
1504           series of operations on two hashes may result in a different order
1505           for each hash.  Any insertion into the hash may change the order,
1506           as will any deletion, with the exception that the most recent key
1507           returned by "each" or "keys" may be deleted without changing the
1508           order.  So long as a given hash is unmodified you may rely on
1509           "keys", "values" and "each" to repeatedly return the same order as
1510           each other.  See "Algorithmic Complexity Attacks" in perlsec for
1511           details on why hash order is randomized.  Aside from the guarantees
1512           provided here the exact details of Perl's hash algorithm and the
1513           hash traversal order are subject to change in any release of Perl.
1514
1515           After "each" has returned all entries from the hash or array, the
1516           next call to "each" returns the empty list in list context and
1517           "undef" in scalar context; the next call following that one
1518           restarts iteration.  Each hash or array has its own internal
1519           iterator, accessed by "each", "keys", and "values".  The iterator
1520           is implicitly reset when "each" has reached the end as just
1521           described; it can be explicitly reset by calling "keys" or "values"
1522           on the hash or array, or by referencing the hash (but not array) in
1523           list context.  If you add or delete a hash's elements while
1524           iterating over it, the effect on the iterator is unspecified; for
1525           example, entries may be skipped or duplicated--so don't do that.
1526           Exception: It is always safe to delete the item most recently
1527           returned by "each", so the following code works properly:
1528
1529               while (my ($key, $value) = each %hash) {
1530                   print $key, "\n";
1531                   delete $hash{$key};   # This is safe
1532               }
1533
1534           Tied hashes may have a different ordering behaviour to perl's hash
1535           implementation.
1536
1537           The iterator used by "each" is attached to the hash or array, and
1538           is shared between all iteration operations applied to the same hash
1539           or array.  Thus all uses of "each" on a single hash or array
1540           advance the same iterator location.  All uses of "each" are also
1541           subject to having the iterator reset by any use of "keys" or
1542           "values" on the same hash or array, or by the hash (but not array)
1543           being referenced in list context.  This makes "each"-based loops
1544           quite fragile: it is easy to arrive at such a loop with the
1545           iterator already part way through the object, or to accidentally
1546           clobber the iterator state during execution of the loop body.  It's
1547           easy enough to explicitly reset the iterator before starting a
1548           loop, but there is no way to insulate the iterator state used by a
1549           loop from the iterator state used by anything else that might
1550           execute during the loop body.  To avoid these problems, use a
1551           "foreach" loop rather than "while"-"each".
1552
1553           This prints out your environment like the printenv(1) program, but
1554           in a different order:
1555
1556               while (my ($key,$value) = each %ENV) {
1557                   print "$key=$value\n";
1558               }
1559
1560           Starting with Perl 5.14, an experimental feature allowed "each" to
1561           take a scalar expression. This experiment has been deemed
1562           unsuccessful, and was removed as of Perl 5.24.
1563
1564           As of Perl 5.18 you can use a bare "each" in a "while" loop, which
1565           will set $_ on every iteration.  If either an "each" expression or
1566           an explicit assignment of an "each" expression to a scalar is used
1567           as a "while"/"for" condition, then the condition actually tests for
1568           definedness of the expression's value, not for its regular truth
1569           value.
1570
1571               while (each %ENV) {
1572                   print "$_=$ENV{$_}\n";
1573               }
1574
1575           To avoid confusing would-be users of your code who are running
1576           earlier versions of Perl with mysterious syntax errors, put this
1577           sort of thing at the top of your file to signal that your code will
1578           work only on Perls of a recent vintage:
1579
1580               use 5.012;  # so keys/values/each work on arrays
1581               use 5.018;  # so each assigns to $_ in a lone while test
1582
1583           See also "keys", "values", and "sort".
1584
1585       eof FILEHANDLE
1586       eof ()
1587       eof Returns 1 if the next read on FILEHANDLE will return end of file or
1588           if FILEHANDLE is not open.  FILEHANDLE may be an expression whose
1589           value gives the real filehandle.  (Note that this function actually
1590           reads a character and then "ungetc"s it, so isn't useful in an
1591           interactive context.)  Do not read from a terminal file (or call
1592           "eof(FILEHANDLE)" on it) after end-of-file is reached.  File types
1593           such as terminals may lose the end-of-file condition if you do.
1594
1595           An "eof" without an argument uses the last file read.  Using
1596           "eof()" with empty parentheses is different.  It refers to the
1597           pseudo file formed from the files listed on the command line and
1598           accessed via the "<>" operator.  Since "<>" isn't explicitly
1599           opened, as a normal filehandle is, an "eof()" before "<>" has been
1600           used will cause @ARGV to be examined to determine if input is
1601           available.   Similarly, an "eof()" after "<>" has returned end-of-
1602           file will assume you are processing another @ARGV list, and if you
1603           haven't set @ARGV, will read input from "STDIN"; see "I/O
1604           Operators" in perlop.
1605
1606           In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect
1607           the end of each file, whereas "eof()" will detect the end of the
1608           very last file only.  Examples:
1609
1610               # reset line numbering on each input file
1611               while (<>) {
1612                   next if /^\s*#/;  # skip comments
1613                   print "$.\t$_";
1614               } continue {
1615                   close ARGV if eof;  # Not eof()!
1616               }
1617
1618               # insert dashes just before last line of last file
1619               while (<>) {
1620                   if (eof()) {  # check for end of last file
1621                       print "--------------\n";
1622                   }
1623                   print;
1624                   last if eof();     # needed if we're reading from a terminal
1625               }
1626
1627           Practical hint: you almost never need to use "eof" in Perl, because
1628           the input operators typically return "undef" when they run out of
1629           data or encounter an error.
1630
1631       eval EXPR
1632       eval BLOCK
1633       eval
1634           "eval" in all its forms is used to execute a little Perl program,
1635           trapping any errors encountered so they don't crash the calling
1636           program.
1637
1638           Plain "eval" with no argument is just "eval EXPR", where the
1639           expression is understood to be contained in $_.  Thus there are
1640           only two real "eval" forms; the one with an EXPR is often called
1641           "string eval".  In a string eval, the value of the expression
1642           (which is itself determined within scalar context) is first parsed,
1643           and if there were no errors, executed as a block within the lexical
1644           context of the current Perl program.  This form is typically used
1645           to delay parsing and subsequent execution of the text of EXPR until
1646           run time.  Note that the value is parsed every time the "eval"
1647           executes.
1648
1649           The other form is called "block eval".  It is less general than
1650           string eval, but the code within the BLOCK is parsed only once (at
1651           the same time the code surrounding the "eval" itself was parsed)
1652           and executed within the context of the current Perl program.  This
1653           form is typically used to trap exceptions more efficiently than the
1654           first, while also providing the benefit of checking the code within
1655           BLOCK at compile time.  BLOCK is parsed and compiled just once.
1656           Since errors are trapped, it often is used to check if a given
1657           feature is available.
1658
1659           In both forms, the value returned is the value of the last
1660           expression evaluated inside the mini-program; a return statement
1661           may also be used, just as with subroutines.  The expression
1662           providing the return value is evaluated in void, scalar, or list
1663           context, depending on the context of the "eval" itself.  See
1664           "wantarray" for more on how the evaluation context can be
1665           determined.
1666
1667           If there is a syntax error or runtime error, or a "die" statement
1668           is executed, "eval" returns "undef" in scalar context, or an empty
1669           list in list context, and $@ is set to the error message.  (Prior
1670           to 5.16, a bug caused "undef" to be returned in list context for
1671           syntax errors, but not for runtime errors.) If there was no error,
1672           $@ is set to the empty string.  A control flow operator like "last"
1673           or "goto" can bypass the setting of $@.  Beware that using "eval"
1674           neither silences Perl from printing warnings to STDERR, nor does it
1675           stuff the text of warning messages into $@.  To do either of those,
1676           you have to use the $SIG{__WARN__} facility, or turn off warnings
1677           inside the BLOCK or EXPR using "no warnings 'all'".  See "warn",
1678           perlvar, and warnings.
1679
1680           Note that, because "eval" traps otherwise-fatal errors, it is
1681           useful for determining whether a particular feature (such as
1682           "socket" or "symlink") is implemented.  It is also Perl's
1683           exception-trapping mechanism, where the "die" operator is used to
1684           raise exceptions.
1685
1686           Before Perl 5.14, the assignment to $@ occurred before restoration
1687           of localized variables, which means that for your code to run on
1688           older versions, a temporary is required if you want to mask some,
1689           but not all errors:
1690
1691            # alter $@ on nefarious repugnancy only
1692            {
1693               my $e;
1694               {
1695                 local $@; # protect existing $@
1696                 eval { test_repugnancy() };
1697                 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1698                 $@ =~ /nefarious/ and $e = $@;
1699               }
1700               die $e if defined $e
1701            }
1702
1703           There are some different considerations for each form:
1704
1705           String eval
1706               Since the return value of EXPR is executed as a block within
1707               the lexical context of the current Perl program, any outer
1708               lexical variables are visible to it, and any package variable
1709               settings or subroutine and format definitions remain
1710               afterwards.
1711
1712               Under the "unicode_eval" feature
1713                   If this feature is enabled (which is the default under a
1714                   "use 5.16" or higher declaration), EXPR is considered to be
1715                   in the same encoding as the surrounding program.  Thus if
1716                   "use utf8" is in effect, the string will be treated as
1717                   being UTF-8 encoded.  Otherwise, the string is considered
1718                   to be a sequence of independent bytes.  Bytes that
1719                   correspond to ASCII-range code points will have their
1720                   normal meanings for operators in the string.  The treatment
1721                   of the other bytes depends on if the "'unicode_strings""
1722                   feature is in effect.
1723
1724                   In a plain "eval" without an EXPR argument, being in
1725                   "use utf8" or not is irrelevant; the UTF-8ness of $_ itself
1726                   determines the behavior.
1727
1728                   Any "use utf8" or "no utf8" declarations within the string
1729                   have no effect, and source filters are forbidden.
1730                   ("unicode_strings", however, can appear within the string.)
1731                   See also the "evalbytes" operator, which works properly
1732                   with source filters.
1733
1734                   Variables defined outside the "eval" and used inside it
1735                   retain their original UTF-8ness.  Everything inside the
1736                   string follows the normal rules for a Perl program with the
1737                   given state of "use utf8".
1738
1739               Outside the "unicode_eval" feature
1740                   In this case, the behavior is problematic and is not so
1741                   easily described.  Here are two bugs that cannot easily be
1742                   fixed without breaking existing programs:
1743
1744                   •   It can lose track of whether something should be
1745                       encoded as UTF-8 or not.
1746
1747                   •   Source filters activated within "eval" leak out into
1748                       whichever file scope is currently being compiled.  To
1749                       give an example with the CPAN module Semi::Semicolons:
1750
1751                        BEGIN { eval "use Semi::Semicolons; # not filtered" }
1752                        # filtered here!
1753
1754                       "evalbytes" fixes that to work the way one would
1755                       expect:
1756
1757                        use feature "evalbytes";
1758                        BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
1759                        # not filtered
1760
1761               Problems can arise if the string expands a scalar containing a
1762               floating point number.  That scalar can expand to letters, such
1763               as "NaN" or "Infinity"; or, within the scope of a "use locale",
1764               the decimal point character may be something other than a dot
1765               (such as a comma).  None of these are likely to parse as you
1766               are likely expecting.
1767
1768               You should be especially careful to remember what's being
1769               looked at when:
1770
1771                   eval $x;        # CASE 1
1772                   eval "$x";      # CASE 2
1773
1774                   eval '$x';      # CASE 3
1775                   eval { $x };    # CASE 4
1776
1777                   eval "\$$x++";  # CASE 5
1778                   $$x++;          # CASE 6
1779
1780               Cases 1 and 2 above behave identically: they run the code
1781               contained in the variable $x.  (Although case 2 has misleading
1782               double quotes making the reader wonder what else might be
1783               happening (nothing is).)  Cases 3 and 4 likewise behave in the
1784               same way: they run the code '$x', which does nothing but return
1785               the value of $x.  (Case 4 is preferred for purely visual
1786               reasons, but it also has the advantage of compiling at compile-
1787               time instead of at run-time.)  Case 5 is a place where normally
1788               you would like to use double quotes, except that in this
1789               particular situation, you can just use symbolic references
1790               instead, as in case 6.
1791
1792               An "eval ''" executed within a subroutine defined in the "DB"
1793               package doesn't see the usual surrounding lexical scope, but
1794               rather the scope of the first non-DB piece of code that called
1795               it.  You don't normally need to worry about this unless you are
1796               writing a Perl debugger.
1797
1798               The final semicolon, if any, may be omitted from the value of
1799               EXPR.
1800
1801           Block eval
1802               If the code to be executed doesn't vary, you may use the eval-
1803               BLOCK form to trap run-time errors without incurring the
1804               penalty of recompiling each time.  The error, if any, is still
1805               returned in $@.  Examples:
1806
1807                   # make divide-by-zero nonfatal
1808                   eval { $answer = $a / $b; }; warn $@ if $@;
1809
1810                   # same thing, but less efficient
1811                   eval '$answer = $a / $b'; warn $@ if $@;
1812
1813                   # a compile-time error
1814                   eval { $answer = }; # WRONG
1815
1816                   # a run-time error
1817                   eval '$answer =';   # sets $@
1818
1819               If you want to trap errors when loading an XS module, some
1820               problems with the binary interface (such as Perl version skew)
1821               may be fatal even with "eval" unless $ENV{PERL_DL_NONLAZY} is
1822               set.  See perlrun.
1823
1824               Using the "eval {}" form as an exception trap in libraries does
1825               have some issues.  Due to the current arguably broken state of
1826               "__DIE__" hooks, you may wish not to trigger any "__DIE__"
1827               hooks that user code may have installed.  You can use the
1828               "local $SIG{__DIE__}" construct for this purpose, as this
1829               example shows:
1830
1831                   # a private exception trap for divide-by-zero
1832                   eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1833                   warn $@ if $@;
1834
1835               This is especially significant, given that "__DIE__" hooks can
1836               call "die" again, which has the effect of changing their error
1837               messages:
1838
1839                   # __DIE__ hooks may modify error messages
1840                   {
1841                      local $SIG{'__DIE__'} =
1842                             sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1843                      eval { die "foo lives here" };
1844                      print $@ if $@;                # prints "bar lives here"
1845                   }
1846
1847               Because this promotes action at a distance, this
1848               counterintuitive behavior may be fixed in a future release.
1849
1850               "eval BLOCK" does not count as a loop, so the loop control
1851               statements "next", "last", or "redo" cannot be used to leave or
1852               restart the block.
1853
1854               The final semicolon, if any, may be omitted from within the
1855               BLOCK.
1856
1857       evalbytes EXPR
1858       evalbytes
1859           This function is similar to a string eval, except it always parses
1860           its argument (or $_ if EXPR is omitted) as a string of independent
1861           bytes.
1862
1863           If called when "use utf8" is in effect, the string will be assumed
1864           to be encoded in UTF-8, and "evalbytes" will make a temporary copy
1865           to work from, downgraded to non-UTF-8.  If this is not possible
1866           (because one or more characters in it require UTF-8), the
1867           "evalbytes" will fail with the error stored in $@.
1868
1869           Bytes that correspond to ASCII-range code points will have their
1870           normal meanings for operators in the string.  The treatment of the
1871           other bytes depends on if the "'unicode_strings"" feature is in
1872           effect.
1873
1874           Of course, variables that are UTF-8 and are referred to in the
1875           string retain that:
1876
1877            my $a = "\x{100}";
1878            evalbytes 'print ord $a, "\n"';
1879
1880           prints
1881
1882            256
1883
1884           and $@ is empty.
1885
1886           Source filters activated within the evaluated code apply to the
1887           code itself.
1888
1889           "evalbytes" is available starting in Perl v5.16.  To access it, you
1890           must say "CORE::evalbytes", but you can omit the "CORE::" if the
1891           "evalbytes" feature is enabled.  This is enabled automatically with
1892           a "use v5.16" (or higher) declaration in the current scope.
1893
1894       exec LIST
1895       exec PROGRAM LIST
1896           The "exec" function executes a system command and never returns;
1897           use "system" instead of "exec" if you want it to return.  It fails
1898           and returns false only if the command does not exist and it is
1899           executed directly instead of via your system's command shell (see
1900           below).
1901
1902           Since it's a common mistake to use "exec" instead of "system", Perl
1903           warns you if "exec" is called in void context and if there is a
1904           following statement that isn't "die", "warn", or "exit" (if
1905           warnings are enabled--but you always do that, right?).  If you
1906           really want to follow an "exec" with some other statement, you can
1907           use one of these styles to avoid the warning:
1908
1909               exec ('foo')   or print STDERR "couldn't exec foo: $!";
1910               { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1911
1912           If there is more than one argument in LIST, this calls execvp(3)
1913           with the arguments in LIST.  If there is only one element in LIST,
1914           the argument is checked for shell metacharacters, and if there are
1915           any, the entire argument is passed to the system's command shell
1916           for parsing (this is "/bin/sh -c" on Unix platforms, but varies on
1917           other platforms).  If there are no shell metacharacters in the
1918           argument, it is split into words and passed directly to "execvp",
1919           which is more efficient.  Examples:
1920
1921               exec '/bin/echo', 'Your arguments are: ', @ARGV;
1922               exec "sort $outfile | uniq";
1923
1924           If you don't really want to execute the first argument, but want to
1925           lie to the program you are executing about its own name, you can
1926           specify the program you actually want to run as an "indirect
1927           object" (without a comma) in front of the LIST, as in "exec PROGRAM
1928           LIST".  (This always forces interpretation of the LIST as a
1929           multivalued list, even if there is only a single scalar in the
1930           list.)  Example:
1931
1932               my $shell = '/bin/csh';
1933               exec $shell '-sh';    # pretend it's a login shell
1934
1935           or, more directly,
1936
1937               exec {'/bin/csh'} '-sh';  # pretend it's a login shell
1938
1939           When the arguments get executed via the system shell, results are
1940           subject to its quirks and capabilities.  See "`STRING`" in perlop
1941           for details.
1942
1943           Using an indirect object with "exec" or "system" is also more
1944           secure.  This usage (which also works fine with "system") forces
1945           interpretation of the arguments as a multivalued list, even if the
1946           list had just one argument.  That way you're safe from the shell
1947           expanding wildcards or splitting up words with whitespace in them.
1948
1949               my @args = ( "echo surprise" );
1950
1951               exec @args;               # subject to shell escapes
1952                                           # if @args == 1
1953               exec { $args[0] } @args;  # safe even with one-arg list
1954
1955           The first version, the one without the indirect object, ran the
1956           echo program, passing it "surprise" an argument.  The second
1957           version didn't; it tried to run a program named "echo surprise",
1958           didn't find it, and set $? to a non-zero value indicating failure.
1959
1960           On Windows, only the "exec PROGRAM LIST" indirect object syntax
1961           will reliably avoid using the shell; "exec LIST", even with more
1962           than one element, will fall back to the shell if the first spawn
1963           fails.
1964
1965           Perl attempts to flush all files opened for output before the exec,
1966           but this may not be supported on some platforms (see perlport).  To
1967           be safe, you may need to set $| ($AUTOFLUSH in English) or call the
1968           "autoflush" method of "IO::Handle" on any open handles to avoid
1969           lost output.
1970
1971           Note that "exec" will not call your "END" blocks, nor will it
1972           invoke "DESTROY" methods on your objects.
1973
1974           Portability issues: "exec" in perlport.
1975
1976       exists EXPR
1977           Given an expression that specifies an element of a hash, returns
1978           true if the specified element in the hash has ever been
1979           initialized, even if the corresponding value is undefined.
1980
1981               print "Exists\n"    if exists $hash{$key};
1982               print "Defined\n"   if defined $hash{$key};
1983               print "True\n"      if $hash{$key};
1984
1985           exists may also be called on array elements, but its behavior is
1986           much less obvious and is strongly tied to the use of "delete" on
1987           arrays.
1988
1989           WARNING: Calling "exists" on array values is strongly discouraged.
1990           The notion of deleting or checking the existence of Perl array
1991           elements is not conceptually coherent, and can lead to surprising
1992           behavior.
1993
1994               print "Exists\n"    if exists $array[$index];
1995               print "Defined\n"   if defined $array[$index];
1996               print "True\n"      if $array[$index];
1997
1998           A hash or array element can be true only if it's defined and
1999           defined only if it exists, but the reverse doesn't necessarily hold
2000           true.
2001
2002           Given an expression that specifies the name of a subroutine,
2003           returns true if the specified subroutine has ever been declared,
2004           even if it is undefined.  Mentioning a subroutine name for exists
2005           or defined does not count as declaring it.  Note that a subroutine
2006           that does not exist may still be callable: its package may have an
2007           "AUTOLOAD" method that makes it spring into existence the first
2008           time that it is called; see perlsub.
2009
2010               print "Exists\n"  if exists &subroutine;
2011               print "Defined\n" if defined &subroutine;
2012
2013           Note that the EXPR can be arbitrarily complicated as long as the
2014           final operation is a hash or array key lookup or subroutine name:
2015
2016               if (exists $ref->{A}->{B}->{$key})  { }
2017               if (exists $hash{A}{B}{$key})       { }
2018
2019               if (exists $ref->{A}->{B}->[$ix])   { }
2020               if (exists $hash{A}{B}[$ix])        { }
2021
2022               if (exists &{$ref->{A}{B}{$key}})   { }
2023
2024           Although the most deeply nested array or hash element will not
2025           spring into existence just because its existence was tested, any
2026           intervening ones will.  Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}"
2027           will spring into existence due to the existence test for the $key
2028           element above.  This happens anywhere the arrow operator is used,
2029           including even here:
2030
2031               undef $ref;
2032               if (exists $ref->{"Some key"})    { }
2033               print $ref;  # prints HASH(0x80d3d5c)
2034
2035           Use of a subroutine call, rather than a subroutine name, as an
2036           argument to "exists" is an error.
2037
2038               exists &sub;    # OK
2039               exists &sub();  # Error
2040
2041       exit EXPR
2042       exit
2043           Evaluates EXPR and exits immediately with that value.    Example:
2044
2045               my $ans = <STDIN>;
2046               exit 0 if $ans =~ /^[Xx]/;
2047
2048           See also "die".  If EXPR is omitted, exits with 0 status.  The only
2049           universally recognized values for EXPR are 0 for success and 1 for
2050           error; other values are subject to interpretation depending on the
2051           environment in which the Perl program is running.  For example,
2052           exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter
2053           will cause the mailer to return the item undelivered, but that's
2054           not true everywhere.
2055
2056           Don't use "exit" to abort a subroutine if there's any chance that
2057           someone might want to trap whatever error happened.  Use "die"
2058           instead, which can be trapped by an "eval".
2059
2060           The "exit" function does not always exit immediately.  It calls any
2061           defined "END" routines first, but these "END" routines may not
2062           themselves abort the exit.  Likewise any object destructors that
2063           need to be called are called before the real exit.  "END" routines
2064           and destructors can change the exit status by modifying $?.  If
2065           this is a problem, you can call "POSIX::_exit($status)" to avoid
2066           "END" and destructor processing.  See perlmod for details.
2067
2068           Portability issues: "exit" in perlport.
2069
2070       exp EXPR
2071       exp Returns e (the natural logarithm base) to the power of EXPR.  If
2072           EXPR is omitted, gives "exp($_)".
2073
2074       fc EXPR
2075       fc  Returns the casefolded version of EXPR.  This is the internal
2076           function implementing the "\F" escape in double-quoted strings.
2077
2078           Casefolding is the process of mapping strings to a form where case
2079           differences are erased; comparing two strings in their casefolded
2080           form is effectively a way of asking if two strings are equal,
2081           regardless of case.
2082
2083           Roughly, if you ever found yourself writing this
2084
2085               lc($this) eq lc($that)    # Wrong!
2086                   # or
2087               uc($this) eq uc($that)    # Also wrong!
2088                   # or
2089               $this =~ /^\Q$that\E\z/i  # Right!
2090
2091           Now you can write
2092
2093               fc($this) eq fc($that)
2094
2095           And get the correct results.
2096
2097           Perl only implements the full form of casefolding, but you can
2098           access the simple folds using "casefold()" in Unicode::UCD and
2099           "prop_invmap()" in Unicode::UCD.  For further information on
2100           casefolding, refer to the Unicode Standard, specifically sections
2101           3.13 "Default Case Operations", 4.2 "Case-Normative", and 5.18
2102           "Case Mappings", available at
2103           <https://www.unicode.org/versions/latest/>, as well as the Case
2104           Charts available at <https://www.unicode.org/charts/case/>.
2105
2106           If EXPR is omitted, uses $_.
2107
2108           This function behaves the same way under various pragmas, such as
2109           within "use feature 'unicode_strings", as "lc" does, with the
2110           single exception of "fc" of LATIN CAPITAL LETTER SHARP S (U+1E9E)
2111           within the scope of "use locale".  The foldcase of this character
2112           would normally be "ss", but as explained in the "lc" section, case
2113           changes that cross the 255/256 boundary are problematic under
2114           locales, and are hence prohibited.  Therefore, this function under
2115           locale returns instead the string "\x{17F}\x{17F}", which is the
2116           LATIN SMALL LETTER LONG S.  Since that character itself folds to
2117           "s", the string of two of them together should be equivalent to a
2118           single U+1E9E when foldcased.
2119
2120           While the Unicode Standard defines two additional forms of
2121           casefolding, one for Turkic languages and one that never maps one
2122           character into multiple characters, these are not provided by the
2123           Perl core.  However, the CPAN module "Unicode::Casing" may be used
2124           to provide an implementation.
2125
2126           "fc" is available only if the "fc" feature is enabled or if it is
2127           prefixed with "CORE::".  The "fc" feature is enabled automatically
2128           with a "use v5.16" (or higher) declaration in the current scope.
2129
2130       fcntl FILEHANDLE,FUNCTION,SCALAR
2131           Implements the fcntl(2) function.  You'll probably have to say
2132
2133               use Fcntl;
2134
2135           first to get the correct constant definitions.  Argument processing
2136           and value returned work just like "ioctl" below.  For example:
2137
2138               use Fcntl;
2139               my $flags = fcntl($filehandle, F_GETFL, 0)
2140                   or die "Can't fcntl F_GETFL: $!";
2141
2142           You don't have to check for "defined" on the return from "fcntl".
2143           Like "ioctl", it maps a 0 return from the system call into "0 but
2144           true" in Perl.  This string is true in boolean context and 0 in
2145           numeric context.  It is also exempt from the normal "Argument "..."
2146           isn't numeric" warnings on improper numeric conversions.
2147
2148           Note that "fcntl" raises an exception if used on a machine that
2149           doesn't implement fcntl(2).  See the Fcntl module or your fcntl(2)
2150           manpage to learn what functions are available on your system.
2151
2152           Here's an example of setting a filehandle named $REMOTE to be non-
2153           blocking at the system level.  You'll have to negotiate $| on your
2154           own, though.
2155
2156               use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2157
2158               my $flags = fcntl($REMOTE, F_GETFL, 0)
2159                   or die "Can't get flags for the socket: $!\n";
2160
2161               fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
2162                   or die "Can't set flags for the socket: $!\n";
2163
2164           Portability issues: "fcntl" in perlport.
2165
2166       __FILE__
2167           A special token that returns the name of the file in which it
2168           occurs.  It can be altered by the mechanism described at "Plain Old
2169           Comments (Not!)" in perlsyn.
2170
2171       fileno FILEHANDLE
2172       fileno DIRHANDLE
2173           Returns the file descriptor for a filehandle or directory handle,
2174           or undefined if the filehandle is not open.  If there is no real
2175           file descriptor at the OS level, as can happen with filehandles
2176           connected to memory objects via "open" with a reference for the
2177           third argument, -1 is returned.
2178
2179           This is mainly useful for constructing bitmaps for "select" and
2180           low-level POSIX tty-handling operations.  If FILEHANDLE is an
2181           expression, the value is taken as an indirect filehandle, generally
2182           its name.
2183
2184           You can use this to find out whether two handles refer to the same
2185           underlying descriptor:
2186
2187               if (fileno($this) != -1 && fileno($this) == fileno($that)) {
2188                   print "\$this and \$that are dups\n";
2189               } elsif (fileno($this) != -1 && fileno($that) != -1) {
2190                   print "\$this and \$that have different " .
2191                       "underlying file descriptors\n";
2192               } else {
2193                   print "At least one of \$this and \$that does " .
2194                       "not have a real file descriptor\n";
2195               }
2196
2197           The behavior of "fileno" on a directory handle depends on the
2198           operating system.  On a system with dirfd(3) or similar, "fileno"
2199           on a directory handle returns the underlying file descriptor
2200           associated with the handle; on systems with no such support, it
2201           returns the undefined value, and sets $! (errno).
2202
2203       flock FILEHANDLE,OPERATION
2204           Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns true
2205           for success, false on failure.  Produces a fatal error if used on a
2206           machine that doesn't implement flock(2), fcntl(2) locking, or
2207           lockf(3).  "flock" is Perl's portable file-locking interface,
2208           although it locks entire files only, not records.
2209
2210           Two potentially non-obvious but traditional "flock" semantics are
2211           that it waits indefinitely until the lock is granted, and that its
2212           locks are merely advisory.  Such discretionary locks are more
2213           flexible, but offer fewer guarantees.  This means that programs
2214           that do not also use "flock" may modify files locked with "flock".
2215           See perlport, your port's specific documentation, and your system-
2216           specific local manpages for details.  It's best to assume
2217           traditional behavior if you're writing portable programs.  (But if
2218           you're not, you should as always feel perfectly free to write for
2219           your own system's idiosyncrasies (sometimes called "features").
2220           Slavish adherence to portability concerns shouldn't get in the way
2221           of your getting your job done.)
2222
2223           OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined
2224           with LOCK_NB.  These constants are traditionally valued 1, 2, 8 and
2225           4, but you can use the symbolic names if you import them from the
2226           Fcntl module, either individually, or as a group using the ":flock"
2227           tag.  LOCK_SH requests a shared lock, LOCK_EX requests an exclusive
2228           lock, and LOCK_UN releases a previously requested lock.  If LOCK_NB
2229           is bitwise-or'ed with LOCK_SH or LOCK_EX, then "flock" returns
2230           immediately rather than blocking waiting for the lock; check the
2231           return status to see if you got it.
2232
2233           To avoid the possibility of miscoordination, Perl now flushes
2234           FILEHANDLE before locking or unlocking it.
2235
2236           Note that the emulation built with lockf(3) doesn't provide shared
2237           locks, and it requires that FILEHANDLE be open with write intent.
2238           These are the semantics that lockf(3) implements.  Most if not all
2239           systems implement lockf(3) in terms of fcntl(2) locking, though, so
2240           the differing semantics shouldn't bite too many people.
2241
2242           Note that the fcntl(2) emulation of flock(3) requires that
2243           FILEHANDLE be open with read intent to use LOCK_SH and requires
2244           that it be open with write intent to use LOCK_EX.
2245
2246           Note also that some versions of "flock" cannot lock things over the
2247           network; you would need to use the more system-specific "fcntl" for
2248           that.  If you like you can force Perl to ignore your system's
2249           flock(2) function, and so provide its own fcntl(2)-based emulation,
2250           by passing the switch "-Ud_flock" to the Configure program when you
2251           configure and build a new Perl.
2252
2253           Here's a mailbox appender for BSD systems.
2254
2255               # import LOCK_* and SEEK_END constants
2256               use Fcntl qw(:flock SEEK_END);
2257
2258               sub lock {
2259                   my ($fh) = @_;
2260                   flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2261                   # and, in case we're running on a very old UNIX
2262                   # variant without the modern O_APPEND semantics...
2263                   seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2264               }
2265
2266               sub unlock {
2267                   my ($fh) = @_;
2268                   flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2269               }
2270
2271               open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2272                   or die "Can't open mailbox: $!";
2273
2274               lock($mbox);
2275               print $mbox $msg,"\n\n";
2276               unlock($mbox);
2277
2278           On systems that support a real flock(2), locks are inherited across
2279           "fork" calls, whereas those that must resort to the more capricious
2280           fcntl(2) function lose their locks, making it seriously harder to
2281           write servers.
2282
2283           See also DB_File for other "flock" examples.
2284
2285           Portability issues: "flock" in perlport.
2286
2287       fork
2288           Does a fork(2) system call to create a new process running the same
2289           program at the same point.  It returns the child pid to the parent
2290           process, 0 to the child process, or "undef" if the fork is
2291           unsuccessful.  File descriptors (and sometimes locks on those
2292           descriptors) are shared, while everything else is copied.  On most
2293           systems supporting fork(2), great care has gone into making it
2294           extremely efficient (for example, using copy-on-write technology on
2295           data pages), making it the dominant paradigm for multitasking over
2296           the last few decades.
2297
2298           Perl attempts to flush all files opened for output before forking
2299           the child process, but this may not be supported on some platforms
2300           (see perlport).  To be safe, you may need to set $| ($AUTOFLUSH in
2301           English) or call the "autoflush" method of "IO::Handle" on any open
2302           handles to avoid duplicate output.
2303
2304           If you "fork" without ever waiting on your children, you will
2305           accumulate zombies.  On some systems, you can avoid this by setting
2306           $SIG{CHLD} to "IGNORE".  See also perlipc for more examples of
2307           forking and reaping moribund children.
2308
2309           Note that if your forked child inherits system file descriptors
2310           like STDIN and STDOUT that are actually connected by a pipe or
2311           socket, even if you exit, then the remote server (such as, say, a
2312           CGI script or a backgrounded job launched from a remote shell)
2313           won't think you're done.  You should reopen those to /dev/null if
2314           it's any issue.
2315
2316           On some platforms such as Windows, where the fork(2) system call is
2317           not available, Perl can be built to emulate "fork" in the Perl
2318           interpreter.  The emulation is designed, at the level of the Perl
2319           program, to be as compatible as possible with the "Unix" fork(2).
2320           However it has limitations that have to be considered in code
2321           intended to be portable.  See perlfork for more details.
2322
2323           Portability issues: "fork" in perlport.
2324
2325       format
2326           Declare a picture format for use by the "write" function.  For
2327           example:
2328
2329               format Something =
2330                   Test: @<<<<<<<< @||||| @>>>>>
2331                         $str,     $%,    '$' . int($num)
2332               .
2333
2334               $str = "widget";
2335               $num = $cost/$quantity;
2336               $~ = 'Something';
2337               write;
2338
2339           See perlform for many details and examples.
2340
2341       formline PICTURE,LIST
2342           This is an internal function used by "format"s, though you may call
2343           it, too.  It formats (see perlform) a list of values according to
2344           the contents of PICTURE, placing the output into the format output
2345           accumulator, $^A (or $ACCUMULATOR in English).  Eventually, when a
2346           "write" is done, the contents of $^A are written to some
2347           filehandle.  You could also read $^A and then set $^A back to "".
2348           Note that a format typically does one "formline" per line of form,
2349           but the "formline" function itself doesn't care how many newlines
2350           are embedded in the PICTURE.  This means that the "~" and "~~"
2351           tokens treat the entire PICTURE as a single line.  You may
2352           therefore need to use multiple formlines to implement a single
2353           record format, just like the "format" compiler.
2354
2355           Be careful if you put double quotes around the picture, because an
2356           "@" character may be taken to mean the beginning of an array name.
2357           "formline" always returns true.  See perlform for other examples.
2358
2359           If you are trying to use this instead of "write" to capture the
2360           output, you may find it easier to open a filehandle to a scalar
2361           ("open my $fh, ">", \$output") and write to that instead.
2362
2363       getc FILEHANDLE
2364       getc
2365           Returns the next character from the input file attached to
2366           FILEHANDLE, or the undefined value at end of file or if there was
2367           an error (in the latter case $! is set).  If FILEHANDLE is omitted,
2368           reads from STDIN.  This is not particularly efficient.  However, it
2369           cannot be used by itself to fetch single characters without waiting
2370           for the user to hit enter.  For that, try something more like:
2371
2372               if ($BSD_STYLE) {
2373                   system "stty cbreak </dev/tty >/dev/tty 2>&1";
2374               }
2375               else {
2376                   system "stty", '-icanon', 'eol', "\001";
2377               }
2378
2379               my $key = getc(STDIN);
2380
2381               if ($BSD_STYLE) {
2382                   system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2383               }
2384               else {
2385                   system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2386               }
2387               print "\n";
2388
2389           Determination of whether $BSD_STYLE should be set is left as an
2390           exercise to the reader.
2391
2392           The "POSIX::getattr" function can do this more portably on systems
2393           purporting POSIX compliance.  See also the "Term::ReadKey" module
2394           on CPAN.
2395
2396       getlogin
2397           This implements the C library function of the same name, which on
2398           most systems returns the current login from /etc/utmp, if any.  If
2399           it returns the empty string, use "getpwuid".
2400
2401               my $login = getlogin || getpwuid($<) || "Kilroy";
2402
2403           Do not consider "getlogin" for authentication: it is not as secure
2404           as "getpwuid".
2405
2406           Portability issues: "getlogin" in perlport.
2407
2408       getpeername SOCKET
2409           Returns the packed sockaddr address of the other end of the SOCKET
2410           connection.
2411
2412               use Socket;
2413               my $hersockaddr    = getpeername($sock);
2414               my ($port, $iaddr) = sockaddr_in($hersockaddr);
2415               my $herhostname    = gethostbyaddr($iaddr, AF_INET);
2416               my $herstraddr     = inet_ntoa($iaddr);
2417
2418       getpgrp PID
2419           Returns the current process group for the specified PID.  Use a PID
2420           of 0 to get the current process group for the current process.
2421           Will raise an exception if used on a machine that doesn't implement
2422           getpgrp(2).  If PID is omitted, returns the process group of the
2423           current process.  Note that the POSIX version of "getpgrp" does not
2424           accept a PID argument, so only "PID==0" is truly portable.
2425
2426           Portability issues: "getpgrp" in perlport.
2427
2428       getppid
2429           Returns the process id of the parent process.
2430
2431           Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2432           around non-POSIX thread semantics the minority of Linux systems
2433           (and Debian GNU/kFreeBSD systems) that used LinuxThreads, this
2434           emulation has since been removed.  See the documentation for $$ for
2435           details.
2436
2437           Portability issues: "getppid" in perlport.
2438
2439       getpriority WHICH,WHO
2440           Returns the current priority for a process, a process group, or a
2441           user.  (See getpriority(2).)  Will raise a fatal exception if used
2442           on a machine that doesn't implement getpriority(2).
2443
2444           "WHICH" can be any of "PRIO_PROCESS", "PRIO_PGRP" or "PRIO_USER"
2445           imported from "RESOURCE CONSTANTS" in POSIX.
2446
2447           Portability issues: "getpriority" in perlport.
2448
2449       getpwnam NAME
2450       getgrnam NAME
2451       gethostbyname NAME
2452       getnetbyname NAME
2453       getprotobyname NAME
2454       getpwuid UID
2455       getgrgid GID
2456       getservbyname NAME,PROTO
2457       gethostbyaddr ADDR,ADDRTYPE
2458       getnetbyaddr ADDR,ADDRTYPE
2459       getprotobynumber NUMBER
2460       getservbyport PORT,PROTO
2461       getpwent
2462       getgrent
2463       gethostent
2464       getnetent
2465       getprotoent
2466       getservent
2467       setpwent
2468       setgrent
2469       sethostent STAYOPEN
2470       setnetent STAYOPEN
2471       setprotoent STAYOPEN
2472       setservent STAYOPEN
2473       endpwent
2474       endgrent
2475       endhostent
2476       endnetent
2477       endprotoent
2478       endservent
2479           These routines are the same as their counterparts in the system C
2480           library.  In list context, the return values from the various get
2481           routines are as follows:
2482
2483            #    0        1          2           3         4
2484            my ( $name,   $passwd,   $gid,       $members  ) = getgr*
2485            my ( $name,   $aliases,  $addrtype,  $net      ) = getnet*
2486            my ( $name,   $aliases,  $port,      $proto    ) = getserv*
2487            my ( $name,   $aliases,  $proto                ) = getproto*
2488            my ( $name,   $aliases,  $addrtype,  $length,  @addrs ) = gethost*
2489            my ( $name,   $passwd,   $uid,       $gid,     $quota,
2490               $comment,  $gcos,     $dir,       $shell,   $expire ) = getpw*
2491            #    5        6          7           8         9
2492
2493           (If the entry doesn't exist, the return value is a single
2494           meaningless true value.)
2495
2496           The exact meaning of the $gcos field varies but it usually contains
2497           the real name of the user (as opposed to the login name) and other
2498           information pertaining to the user.  Beware, however, that in many
2499           system users are able to change this information and therefore it
2500           cannot be trusted and therefore the $gcos is tainted (see perlsec).
2501           The $passwd and $shell, user's encrypted password and login shell,
2502           are also tainted, for the same reason.
2503
2504           In scalar context, you get the name, unless the function was a
2505           lookup by name, in which case you get the other thing, whatever it
2506           is.  (If the entry doesn't exist you get the undefined value.)  For
2507           example:
2508
2509               my $uid   = getpwnam($name);
2510               my $name  = getpwuid($num);
2511               my $name  = getpwent();
2512               my $gid   = getgrnam($name);
2513               my $name  = getgrgid($num);
2514               my $name  = getgrent();
2515               # etc.
2516
2517           In getpw*() the fields $quota, $comment, and $expire are special in
2518           that they are unsupported on many systems.  If the $quota is
2519           unsupported, it is an empty scalar.  If it is supported, it usually
2520           encodes the disk quota.  If the $comment field is unsupported, it
2521           is an empty scalar.  If it is supported it usually encodes some
2522           administrative comment about the user.  In some systems the $quota
2523           field may be $change or $age, fields that have to do with password
2524           aging.  In some systems the $comment field may be $class.  The
2525           $expire field, if present, encodes the expiration period of the
2526           account or the password.  For the availability and the exact
2527           meaning of these fields in your system, please consult getpwnam(3)
2528           and your system's pwd.h file.  You can also find out from within
2529           Perl what your $quota and $comment fields mean and whether you have
2530           the $expire field by using the "Config" module and the values
2531           "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and
2532           "d_pwexpire".  Shadow password files are supported only if your
2533           vendor has implemented them in the intuitive fashion that calling
2534           the regular C library routines gets the shadow versions if you're
2535           running under privilege or if there exists the shadow(3) functions
2536           as found in System V (this includes Solaris and Linux).  Those
2537           systems that implement a proprietary shadow password facility are
2538           unlikely to be supported.
2539
2540           The $members value returned by getgr*() is a space-separated list
2541           of the login names of the members of the group.
2542
2543           For the gethost*() functions, if the "h_errno" variable is
2544           supported in C, it will be returned to you via $? if the function
2545           call fails.  The @addrs value returned by a successful call is a
2546           list of raw addresses returned by the corresponding library call.
2547           In the Internet domain, each address is four bytes long; you can
2548           unpack it by saying something like:
2549
2550               my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
2551
2552           The Socket library makes this slightly easier:
2553
2554               use Socket;
2555               my $iaddr = inet_aton("127.1"); # or whatever address
2556               my $name  = gethostbyaddr($iaddr, AF_INET);
2557
2558               # or going the other way
2559               my $straddr = inet_ntoa($iaddr);
2560
2561           In the opposite way, to resolve a hostname to the IP address you
2562           can write this:
2563
2564               use Socket;
2565               my $packed_ip = gethostbyname("www.perl.org");
2566               my $ip_address;
2567               if (defined $packed_ip) {
2568                   $ip_address = inet_ntoa($packed_ip);
2569               }
2570
2571           Make sure "gethostbyname" is called in SCALAR context and that its
2572           return value is checked for definedness.
2573
2574           The "getprotobynumber" function, even though it only takes one
2575           argument, has the precedence of a list operator, so beware:
2576
2577               getprotobynumber $number eq 'icmp'   # WRONG
2578               getprotobynumber($number eq 'icmp')  # actually means this
2579               getprotobynumber($number) eq 'icmp'  # better this way
2580
2581           If you get tired of remembering which element of the return list
2582           contains which return value, by-name interfaces are provided in
2583           standard modules: "File::stat", "Net::hostent", "Net::netent",
2584           "Net::protoent", "Net::servent", "Time::gmtime", "Time::localtime",
2585           and "User::grent".  These override the normal built-ins, supplying
2586           versions that return objects with the appropriate names for each
2587           field.  For example:
2588
2589              use File::stat;
2590              use User::pwent;
2591              my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2592
2593           Even though it looks as though they're the same method calls (uid),
2594           they aren't, because a "File::stat" object is different from a
2595           "User::pwent" object.
2596
2597           Many of these functions are not safe in a multi-threaded
2598           environment where more than one thread can be using them.  In
2599           particular, functions like "getpwent()" iterate per-process and not
2600           per-thread, so if two threads are simultaneously iterating, neither
2601           will get all the records.
2602
2603           Some systems have thread-safe versions of some of the functions,
2604           such as "getpwnam_r()" instead of "getpwnam()".  There, Perl
2605           automatically and invisibly substitutes the thread-safe version,
2606           without notice.  This means that code that safely runs on some
2607           systems can fail on others that lack the thread-safe versions.
2608
2609           Portability issues: "getpwnam" in perlport to "endservent" in
2610           perlport.
2611
2612       getsockname SOCKET
2613           Returns the packed sockaddr address of this end of the SOCKET
2614           connection, in case you don't know the address because you have
2615           several different IPs that the connection might have come in on.
2616
2617               use Socket;
2618               my $mysockaddr = getsockname($sock);
2619               my ($port, $myaddr) = sockaddr_in($mysockaddr);
2620               printf "Connect to %s [%s]\n",
2621                  scalar gethostbyaddr($myaddr, AF_INET),
2622                  inet_ntoa($myaddr);
2623
2624       getsockopt SOCKET,LEVEL,OPTNAME
2625           Queries the option named OPTNAME associated with SOCKET at a given
2626           LEVEL.  Options may exist at multiple protocol levels depending on
2627           the socket type, but at least the uppermost socket level SOL_SOCKET
2628           (defined in the "Socket" module) will exist.  To query options at
2629           another level the protocol number of the appropriate protocol
2630           controlling the option should be supplied.  For example, to
2631           indicate that an option is to be interpreted by the TCP protocol,
2632           LEVEL should be set to the protocol number of TCP, which you can
2633           get using "getprotobyname".
2634
2635           The function returns a packed string representing the requested
2636           socket option, or "undef" on error, with the reason for the error
2637           placed in $!.  Just what is in the packed string depends on LEVEL
2638           and OPTNAME; consult getsockopt(2) for details.  A common case is
2639           that the option is an integer, in which case the result is a packed
2640           integer, which you can decode using "unpack" with the "i" (or "I")
2641           format.
2642
2643           Here's an example to test whether Nagle's algorithm is enabled on a
2644           socket:
2645
2646               use Socket qw(:all);
2647
2648               defined(my $tcp = getprotobyname("tcp"))
2649                   or die "Could not determine the protocol number for tcp";
2650               # my $tcp = IPPROTO_TCP; # Alternative
2651               my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2652                   or die "getsockopt TCP_NODELAY: $!";
2653               my $nodelay = unpack("I", $packed);
2654               print "Nagle's algorithm is turned ",
2655                      $nodelay ? "off\n" : "on\n";
2656
2657           Portability issues: "getsockopt" in perlport.
2658
2659       glob EXPR
2660       glob
2661           In list context, returns a (possibly empty) list of filename
2662           expansions on the value of EXPR such as the standard Unix shell
2663           /bin/csh would do.  In scalar context, glob iterates through such
2664           filename expansions, returning undef when the list is exhausted.
2665           This is the internal function implementing the "<*.c>" operator,
2666           but you can use it directly.  If EXPR is omitted, $_ is used.  The
2667           "<*.c>" operator is discussed in more detail in "I/O Operators" in
2668           perlop.
2669
2670           Note that "glob" splits its arguments on whitespace and treats each
2671           segment as separate pattern.  As such, "glob("*.c *.h")" matches
2672           all files with a .c or .h extension.  The expression "glob(".* *")"
2673           matches all files in the current working directory.  If you want to
2674           glob filenames that might contain whitespace, you'll have to use
2675           extra quotes around the spacey filename to protect it.  For
2676           example, to glob filenames that have an "e" followed by a space
2677           followed by an "f", use one of:
2678
2679               my @spacies = <"*e f*">;
2680               my @spacies = glob '"*e f*"';
2681               my @spacies = glob q("*e f*");
2682
2683           If you had to get a variable through, you could do this:
2684
2685               my @spacies = glob "'*${var}e f*'";
2686               my @spacies = glob qq("*${var}e f*");
2687
2688           If non-empty braces are the only wildcard characters used in the
2689           "glob", no filenames are matched, but potentially many strings are
2690           returned.  For example, this produces nine strings, one for each
2691           pairing of fruits and colors:
2692
2693               my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2694
2695           This operator is implemented using the standard "File::Glob"
2696           extension.  See File::Glob for details, including "bsd_glob", which
2697           does not treat whitespace as a pattern separator.
2698
2699           If a "glob" expression is used as the condition of a "while" or
2700           "for" loop, then it will be implicitly assigned to $_.  If either a
2701           "glob" expression or an explicit assignment of a "glob" expression
2702           to a scalar is used as a "while"/"for" condition, then the
2703           condition actually tests for definedness of the expression's value,
2704           not for its regular truth value.
2705
2706           Portability issues: "glob" in perlport.
2707
2708       gmtime EXPR
2709       gmtime
2710           Works just like "localtime", but the returned values are localized
2711           for the standard Greenwich time zone.
2712
2713           Note: When called in list context, $isdst, the last value returned
2714           by gmtime, is always 0.  There is no Daylight Saving Time in GMT.
2715
2716           Portability issues: "gmtime" in perlport.
2717
2718       goto LABEL
2719       goto EXPR
2720       goto &NAME
2721           The "goto LABEL" form finds the statement labeled with LABEL and
2722           resumes execution there.  It can't be used to get out of a block or
2723           subroutine given to "sort".  It can be used to go almost anywhere
2724           else within the dynamic scope, including out of subroutines, but
2725           it's usually better to use some other construct such as "last" or
2726           "die".  The author of Perl has never felt the need to use this form
2727           of "goto" (in Perl, that is; C is another matter).  (The difference
2728           is that C does not offer named loops combined with loop control.
2729           Perl does, and this replaces most structured uses of "goto" in
2730           other languages.)
2731
2732           The "goto EXPR" form expects to evaluate "EXPR" to a code reference
2733           or a label name.  If it evaluates to a code reference, it will be
2734           handled like "goto &NAME", below.  This is especially useful for
2735           implementing tail recursion via "goto __SUB__".
2736
2737           If the expression evaluates to a label name, its scope will be
2738           resolved dynamically.  This allows for computed "goto"s per
2739           FORTRAN, but isn't necessarily recommended if you're optimizing for
2740           maintainability:
2741
2742               goto ("FOO", "BAR", "GLARCH")[$i];
2743
2744           As shown in this example, "goto EXPR" is exempt from the "looks
2745           like a function" rule.  A pair of parentheses following it does not
2746           (necessarily) delimit its argument.  "goto("NE")."XT"" is
2747           equivalent to "goto NEXT".  Also, unlike most named operators, this
2748           has the same precedence as assignment.
2749
2750           Use of "goto LABEL" or "goto EXPR" to jump into a construct is
2751           deprecated and will issue a warning.  Even then, it may not be used
2752           to go into any construct that requires initialization, such as a
2753           subroutine, a "foreach" loop, or a "given" block.  In general, it
2754           may not be used to jump into the parameter of a binary or list
2755           operator, but it may be used to jump into the first parameter of a
2756           binary operator.  (The "=" assignment operator's "first" operand is
2757           its right-hand operand.)  It also can't be used to go into a
2758           construct that is optimized away.
2759
2760           The "goto &NAME" form is quite different from the other forms of
2761           "goto".  In fact, it isn't a goto in the normal sense at all, and
2762           doesn't have the stigma associated with other gotos.  Instead, it
2763           exits the current subroutine (losing any changes set by "local")
2764           and immediately calls in its place the named subroutine using the
2765           current value of @_.  This is used by "AUTOLOAD" subroutines that
2766           wish to load another subroutine and then pretend that the other
2767           subroutine had been called in the first place (except that any
2768           modifications to @_ in the current subroutine are propagated to the
2769           other subroutine.) After the "goto", not even "caller" will be able
2770           to tell that this routine was called first.
2771
2772           NAME needn't be the name of a subroutine; it can be a scalar
2773           variable containing a code reference or a block that evaluates to a
2774           code reference.
2775
2776       grep BLOCK LIST
2777       grep EXPR,LIST
2778           This is similar in spirit to, but not the same as, grep(1) and its
2779           relatives.  In particular, it is not limited to using regular
2780           expressions.
2781
2782           Evaluates the BLOCK or EXPR for each element of LIST (locally
2783           setting $_ to each element) and returns the list value consisting
2784           of those elements for which the expression evaluated to true.  In
2785           scalar context, returns the number of times the expression was
2786           true.
2787
2788               my @foo = grep(!/^#/, @bar);    # weed out comments
2789
2790           or equivalently,
2791
2792               my @foo = grep {!/^#/} @bar;    # weed out comments
2793
2794           Note that $_ is an alias to the list value, so it can be used to
2795           modify the elements of the LIST.  While this is useful and
2796           supported, it can cause bizarre results if the elements of LIST are
2797           not variables.  Similarly, grep returns aliases into the original
2798           list, much as a for loop's index variable aliases the list
2799           elements.  That is, modifying an element of a list returned by grep
2800           (for example, in a "foreach", "map" or another "grep") actually
2801           modifies the element in the original list.  This is usually
2802           something to be avoided when writing clear code.
2803
2804           See also "map" for a list composed of the results of the BLOCK or
2805           EXPR.
2806
2807       hex EXPR
2808       hex Interprets EXPR as a hex string and returns the corresponding
2809           numeric value.  If EXPR is omitted, uses $_.
2810
2811               print hex '0xAf'; # prints '175'
2812               print hex 'aF';   # same
2813               $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
2814
2815           A hex string consists of hex digits and an optional "0x" or "x"
2816           prefix.  Each hex digit may be preceded by a single underscore,
2817           which will be ignored.  Any other character triggers a warning and
2818           causes the rest of the string to be ignored (even leading
2819           whitespace, unlike "oct").  Only integers can be represented, and
2820           integer overflow triggers a warning.
2821
2822           To convert strings that might start with any of 0, "0x", or "0b",
2823           see "oct".  To present something as hex, look into "printf",
2824           "sprintf", and "unpack".
2825
2826       import LIST
2827           There is no builtin "import" function.  It is just an ordinary
2828           method (subroutine) defined (or inherited) by modules that wish to
2829           export names to another module.  The "use" function calls the
2830           "import" method for the package used.  See also "use", perlmod, and
2831           Exporter.
2832
2833       index STR,SUBSTR,POSITION
2834       index STR,SUBSTR
2835           The index function searches for one string within another, but
2836           without the wildcard-like behavior of a full regular-expression
2837           pattern match.  It returns the position of the first occurrence of
2838           SUBSTR in STR at or after POSITION.  If POSITION is omitted, starts
2839           searching from the beginning of the string.  POSITION before the
2840           beginning of the string or after its end is treated as if it were
2841           the beginning or the end, respectively.  POSITION and the return
2842           value are based at zero.  If the substring is not found, "index"
2843           returns -1.
2844
2845           Find characters or strings:
2846
2847               index("Perl is great", "P");     # Returns 0
2848               index("Perl is great", "g");     # Returns 8
2849               index("Perl is great", "great"); # Also returns 8
2850
2851           Attempting to find something not there:
2852
2853               index("Perl is great", "Z");     # Returns -1 (not found)
2854
2855           Using an offset to find the second occurrence:
2856
2857               index("Perl is great", "e", 5);  # Returns 10
2858
2859       int EXPR
2860       int Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.
2861           You should not use this function for rounding: one because it
2862           truncates towards 0, and two because machine representations of
2863           floating-point numbers can sometimes produce counterintuitive
2864           results.  For example, "int(-6.725/0.025)" produces -268 rather
2865           than the correct -269; that's because it's really more like
2866           -268.99999999999994315658 instead.  Usually, the "sprintf",
2867           "printf", or the "POSIX::floor" and "POSIX::ceil" functions will
2868           serve you better than will "int".
2869
2870       ioctl FILEHANDLE,FUNCTION,SCALAR
2871           Implements the ioctl(2) function.  You'll probably first have to
2872           say
2873
2874               require "sys/ioctl.ph";  # probably in
2875                                        # $Config{archlib}/sys/ioctl.ph
2876
2877           to get the correct function definitions.  If sys/ioctl.ph doesn't
2878           exist or doesn't have the correct definitions you'll have to roll
2879           your own, based on your C header files such as <sys/ioctl.h>.
2880           (There is a Perl script called h2ph that comes with the Perl kit
2881           that may help you in this, but it's nontrivial.)  SCALAR will be
2882           read and/or written depending on the FUNCTION; a C pointer to the
2883           string value of SCALAR will be passed as the third argument of the
2884           actual "ioctl" call.  (If SCALAR has no string value but does have
2885           a numeric value, that value will be passed rather than a pointer to
2886           the string value.  To guarantee this to be true, add a 0 to the
2887           scalar before using it.)  The "pack" and "unpack" functions may be
2888           needed to manipulate the values of structures used by "ioctl".
2889
2890           The return value of "ioctl" (and "fcntl") is as follows:
2891
2892               if OS returns:      then Perl returns:
2893                   -1               undefined value
2894                    0              string "0 but true"
2895               anything else           that number
2896
2897           Thus Perl returns true on success and false on failure, yet you can
2898           still easily determine the actual value returned by the operating
2899           system:
2900
2901               my $retval = ioctl(...) || -1;
2902               printf "System returned %d\n", $retval;
2903
2904           The special string "0 but true" is exempt from "Argument "..."
2905           isn't numeric" warnings on improper numeric conversions.
2906
2907           Portability issues: "ioctl" in perlport.
2908
2909       join EXPR,LIST
2910           Joins the separate strings of LIST into a single string with fields
2911           separated by the value of EXPR, and returns that new string.
2912           Example:
2913
2914              my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2915
2916           Beware that unlike "split", "join" doesn't take a pattern as its
2917           first argument.  Compare "split".
2918
2919       keys HASH
2920       keys ARRAY
2921           Called in list context, returns a list consisting of all the keys
2922           of the named hash, or in Perl 5.12 or later only, the indices of an
2923           array.  Perl releases prior to 5.12 will produce a syntax error if
2924           you try to use an array argument.  In scalar context, returns the
2925           number of keys or indices.
2926
2927           Hash entries are returned in an apparently random order.  The
2928           actual random order is specific to a given hash; the exact same
2929           series of operations on two hashes may result in a different order
2930           for each hash.  Any insertion into the hash may change the order,
2931           as will any deletion, with the exception that the most recent key
2932           returned by "each" or "keys" may be deleted without changing the
2933           order.  So long as a given hash is unmodified you may rely on
2934           "keys", "values" and "each" to repeatedly return the same order as
2935           each other.  See "Algorithmic Complexity Attacks" in perlsec for
2936           details on why hash order is randomized.  Aside from the guarantees
2937           provided here the exact details of Perl's hash algorithm and the
2938           hash traversal order are subject to change in any release of Perl.
2939           Tied hashes may behave differently to Perl's hashes with respect to
2940           changes in order on insertion and deletion of items.
2941
2942           As a side effect, calling "keys" resets the internal iterator of
2943           the HASH or ARRAY (see "each") before yielding the keys.  In
2944           particular, calling "keys" in void context resets the iterator with
2945           no other overhead.
2946
2947           Here is yet another way to print your environment:
2948
2949               my @keys = keys %ENV;
2950               my @values = values %ENV;
2951               while (@keys) {
2952                   print pop(@keys), '=', pop(@values), "\n";
2953               }
2954
2955           or how about sorted by key:
2956
2957               foreach my $key (sort(keys %ENV)) {
2958                   print $key, '=', $ENV{$key}, "\n";
2959               }
2960
2961           The returned values are copies of the original keys in the hash, so
2962           modifying them will not affect the original hash.  Compare
2963           "values".
2964
2965           To sort a hash by value, you'll need to use a "sort" function.
2966           Here's a descending numeric sort of a hash by its values:
2967
2968               foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2969                   printf "%4d %s\n", $hash{$key}, $key;
2970               }
2971
2972           Used as an lvalue, "keys" allows you to increase the number of hash
2973           buckets allocated for the given hash.  This can gain you a measure
2974           of efficiency if you know the hash is going to get big.  (This is
2975           similar to pre-extending an array by assigning a larger number to
2976           $#array.)  If you say
2977
2978               keys %hash = 200;
2979
2980           then %hash will have at least 200 buckets allocated for it--256 of
2981           them, in fact, since it rounds up to the next power of two.  These
2982           buckets will be retained even if you do "%hash = ()", use "undef
2983           %hash" if you want to free the storage while %hash is still in
2984           scope.  You can't shrink the number of buckets allocated for the
2985           hash using "keys" in this way (but you needn't worry about doing
2986           this by accident, as trying has no effect).  "keys @array" in an
2987           lvalue context is a syntax error.
2988
2989           Starting with Perl 5.14, an experimental feature allowed "keys" to
2990           take a scalar expression. This experiment has been deemed
2991           unsuccessful, and was removed as of Perl 5.24.
2992
2993           To avoid confusing would-be users of your code who are running
2994           earlier versions of Perl with mysterious syntax errors, put this
2995           sort of thing at the top of your file to signal that your code will
2996           work only on Perls of a recent vintage:
2997
2998               use 5.012;  # so keys/values/each work on arrays
2999
3000           See also "each", "values", and "sort".
3001
3002       kill SIGNAL, LIST
3003       kill SIGNAL
3004           Sends a signal to a list of processes.  Returns the number of
3005           arguments that were successfully used to signal (which is not
3006           necessarily the same as the number of processes actually killed,
3007           e.g. where a process group is killed).
3008
3009               my $cnt = kill 'HUP', $child1, $child2;
3010               kill 'KILL', @goners;
3011
3012           SIGNAL may be either a signal name (a string) or a signal number.
3013           A signal name may start with a "SIG" prefix, thus "FOO" and
3014           "SIGFOO" refer to the same signal.  The string form of SIGNAL is
3015           recommended for portability because the same signal may have
3016           different numbers in different operating systems.
3017
3018           A list of signal names supported by the current platform can be
3019           found in $Config{sig_name}, which is provided by the "Config"
3020           module.  See Config for more details.
3021
3022           A negative signal name is the same as a negative signal number,
3023           killing process groups instead of processes.  For example, "kill
3024           '-KILL', $pgrp" and "kill -9, $pgrp" will send "SIGKILL" to the
3025           entire process group specified.  That means you usually want to use
3026           positive not negative signals.
3027
3028           If SIGNAL is either the number 0 or the string "ZERO" (or
3029           "SIGZERO"), no signal is sent to the process, but "kill" checks
3030           whether it's possible to send a signal to it (that means, to be
3031           brief, that the process is owned by the same user, or we are the
3032           super-user).  This is useful to check that a child process is still
3033           alive (even if only as a zombie) and hasn't changed its UID.  See
3034           perlport for notes on the portability of this construct.
3035
3036           The behavior of kill when a PROCESS number is zero or negative
3037           depends on the operating system.  For example, on POSIX-conforming
3038           systems, zero will signal the current process group, -1 will signal
3039           all processes, and any other negative PROCESS number will act as a
3040           negative signal number and kill the entire process group specified.
3041
3042           If both the SIGNAL and the PROCESS are negative, the results are
3043           undefined.  A warning may be produced in a future version.
3044
3045           See "Signals" in perlipc for more details.
3046
3047           On some platforms such as Windows where the fork(2) system call is
3048           not available, Perl can be built to emulate "fork" at the
3049           interpreter level.  This emulation has limitations related to kill
3050           that have to be considered, for code running on Windows and in code
3051           intended to be portable.
3052
3053           See perlfork for more details.
3054
3055           If there is no LIST of processes, no signal is sent, and the return
3056           value is 0.  This form is sometimes used, however, because it
3057           causes tainting checks to be run.  But see "Laundering and
3058           Detecting Tainted Data" in perlsec.
3059
3060           Portability issues: "kill" in perlport.
3061
3062       last LABEL
3063       last EXPR
3064       last
3065           The "last" command is like the "break" statement in C (as used in
3066           loops); it immediately exits the loop in question.  If the LABEL is
3067           omitted, the command refers to the innermost enclosing loop.  The
3068           "last EXPR" form, available starting in Perl 5.18.0, allows a label
3069           name to be computed at run time, and is otherwise identical to
3070           "last LABEL".  The "continue" block, if any, is not executed:
3071
3072               LINE: while (<STDIN>) {
3073                   last LINE if /^$/;  # exit when done with header
3074                   #...
3075               }
3076
3077           "last" cannot return a value from a block that typically returns a
3078           value, such as "eval {}", "sub {}", or "do {}". It will perform its
3079           flow control behavior, which precludes any return value. It should
3080           not be used to exit a "grep" or "map" operation.
3081
3082           Note that a block by itself is semantically identical to a loop
3083           that executes once.  Thus "last" can be used to effect an early
3084           exit out of such a block.
3085
3086           See also "continue" for an illustration of how "last", "next", and
3087           "redo" work.
3088
3089           Unlike most named operators, this has the same precedence as
3090           assignment.  It is also exempt from the looks-like-a-function rule,
3091           so "last ("foo")."bar"" will cause "bar" to be part of the argument
3092           to "last".
3093
3094       lc EXPR
3095       lc  Returns a lowercased version of EXPR.  This is the internal
3096           function implementing the "\L" escape in double-quoted strings.
3097
3098           If EXPR is omitted, uses $_.
3099
3100           What gets returned depends on several factors:
3101
3102           If "use bytes" is in effect:
3103               The results follow ASCII rules.  Only the characters "A-Z"
3104               change, to "a-z" respectively.
3105
3106           Otherwise, if "use locale" for "LC_CTYPE" is in effect:
3107               Respects current "LC_CTYPE" locale for code points < 256; and
3108               uses Unicode rules for the remaining code points (this last can
3109               only happen if the UTF8 flag is also set).  See perllocale.
3110
3111               Starting in v5.20, Perl uses full Unicode rules if the locale
3112               is UTF-8.  Otherwise, there is a deficiency in this scheme,
3113               which is that case changes that cross the 255/256 boundary are
3114               not well-defined.  For example, the lower case of LATIN CAPITAL
3115               LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3116               platforms).   But under "use locale" (prior to v5.20 or not a
3117               UTF-8 locale), the lower case of U+1E9E is itself, because 0xDF
3118               may not be LATIN SMALL LETTER SHARP S in the current locale,
3119               and Perl has no way of knowing if that character even exists in
3120               the locale, much less what code point it is.  Perl returns a
3121               result that is above 255 (almost always the input character
3122               unchanged), for all instances (and there aren't many) where the
3123               255/256 boundary would otherwise be crossed; and starting in
3124               v5.22, it raises a locale warning.
3125
3126           Otherwise, If EXPR has the UTF8 flag set:
3127               Unicode rules are used for the case change.
3128
3129           Otherwise, if "use feature 'unicode_strings'" or "use locale
3130           ':not_characters'" is in effect:
3131               Unicode rules are used for the case change.
3132
3133           Otherwise:
3134               ASCII rules are used for the case change.  The lowercase of any
3135               character outside the ASCII range is the character itself.
3136
3137       lcfirst EXPR
3138       lcfirst
3139           Returns the value of EXPR with the first character lowercased.
3140           This is the internal function implementing the "\l" escape in
3141           double-quoted strings.
3142
3143           If EXPR is omitted, uses $_.
3144
3145           This function behaves the same way under various pragmas, such as
3146           in a locale, as "lc" does.
3147
3148       length EXPR
3149       length
3150           Returns the length in characters of the value of EXPR.  If EXPR is
3151           omitted, returns the length of $_.  If EXPR is undefined, returns
3152           "undef".
3153
3154           This function cannot be used on an entire array or hash to find out
3155           how many elements these have.  For that, use "scalar @array" and
3156           "scalar keys %hash", respectively.
3157
3158           Like all Perl character operations, "length" normally deals in
3159           logical characters, not physical bytes.  For how many bytes a
3160           string encoded as UTF-8 would take up, use
3161           "length(Encode::encode('UTF-8', EXPR))" (you'll have to "use
3162           Encode" first).  See Encode and perlunicode.
3163
3164       __LINE__
3165           A special token that compiles to the current line number.  It can
3166           be altered by the mechanism described at "Plain Old Comments
3167           (Not!)" in perlsyn.
3168
3169       link OLDFILE,NEWFILE
3170           Creates a new filename linked to the old filename.  Returns true
3171           for success, false otherwise.
3172
3173           Portability issues: "link" in perlport.
3174
3175       listen SOCKET,QUEUESIZE
3176           Does the same thing that the listen(2) system call does.  Returns
3177           true if it succeeded, false otherwise.  See the example in
3178           "Sockets: Client/Server Communication" in perlipc.
3179
3180       local EXPR
3181           You really probably want to be using "my" instead, because "local"
3182           isn't what most people think of as "local".  See "Private Variables
3183           via my()" in perlsub for details.
3184
3185           A local modifies the listed variables to be local to the enclosing
3186           block, file, or eval.  If more than one value is listed, the list
3187           must be placed in parentheses.  See "Temporary Values via local()"
3188           in perlsub for details, including issues with tied arrays and
3189           hashes.
3190
3191           The "delete local EXPR" construct can also be used to localize the
3192           deletion of array/hash elements to the current block.  See
3193           "Localized deletion of elements of composite types" in perlsub.
3194
3195       localtime EXPR
3196       localtime
3197           Converts a time as returned by the time function to a 9-element
3198           list with the time analyzed for the local time zone.  Typically
3199           used as follows:
3200
3201               #     0    1    2     3     4    5     6     7     8
3202               my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3203                                                           localtime(time);
3204
3205           All list elements are numeric and come straight out of the C
3206           `struct tm'.  $sec, $min, and $hour are the seconds, minutes, and
3207           hours of the specified time.
3208
3209           $mday is the day of the month and $mon the month in the range
3210           0..11, with 0 indicating January and 11 indicating December.  This
3211           makes it easy to get a month name from a list:
3212
3213               my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3214               print "$abbr[$mon] $mday";
3215               # $mon=9, $mday=18 gives "Oct 18"
3216
3217           $year contains the number of years since 1900.  To get a 4-digit
3218           year write:
3219
3220               $year += 1900;
3221
3222           To get the last two digits of the year (e.g., "01" in 2001) do:
3223
3224               $year = sprintf("%02d", $year % 100);
3225
3226           $wday is the day of the week, with 0 indicating Sunday and 3
3227           indicating Wednesday.  $yday is the day of the year, in the range
3228           0..364 (or 0..365 in leap years.)
3229
3230           $isdst is true if the specified time occurs when Daylight Saving
3231           Time is in effect, false otherwise.
3232
3233           If EXPR is omitted, "localtime" uses the current time (as returned
3234           by "time").
3235
3236           In scalar context, "localtime" returns the ctime(3) value:
3237
3238            my $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"
3239
3240           This scalar value is always in English, and is not locale-
3241           dependent.  To get similar but locale-dependent date strings, try
3242           for example:
3243
3244            use POSIX qw(strftime);
3245            my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3246            # or for GMT formatted appropriately for your locale:
3247            my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3248
3249           C$now_string> will be formatted according to the current LC_TIME
3250           locale the program or thread is running in.  See perllocale for how
3251           to set up and change that locale.  Note that %a and %b, the short
3252           forms of the day of the week and the month of the year, may not
3253           necessarily be three characters wide.
3254
3255           The Time::gmtime and Time::localtime modules provide a convenient,
3256           by-name access mechanism to the "gmtime" and "localtime" functions,
3257           respectively.
3258
3259           For a comprehensive date and time representation look at the
3260           DateTime module on CPAN.
3261
3262           For GMT instead of local time use the "gmtime" builtin.
3263
3264           See also the "Time::Local" module (for converting seconds, minutes,
3265           hours, and such back to the integer value returned by "time"), and
3266           the POSIX module's "mktime" function.
3267
3268           Portability issues: "localtime" in perlport.
3269
3270       lock THING
3271           This function places an advisory lock on a shared variable or
3272           referenced object contained in THING until the lock goes out of
3273           scope.
3274
3275           The value returned is the scalar itself, if the argument is a
3276           scalar, or a reference, if the argument is a hash, array or
3277           subroutine.
3278
3279           "lock" is a "weak keyword"; this means that if you've defined a
3280           function by this name (before any calls to it), that function will
3281           be called instead.  If you are not under "use threads::shared" this
3282           does nothing.  See threads::shared.
3283
3284       log EXPR
3285       log Returns the natural logarithm (base e) of EXPR.  If EXPR is
3286           omitted, returns the log of $_.  To get the log of another base,
3287           use basic algebra: The base-N log of a number is equal to the
3288           natural log of that number divided by the natural log of N.  For
3289           example:
3290
3291               sub log10 {
3292                   my $n = shift;
3293                   return log($n)/log(10);
3294               }
3295
3296           See also "exp" for the inverse operation.
3297
3298       lstat FILEHANDLE
3299       lstat EXPR
3300       lstat DIRHANDLE
3301       lstat
3302           Does the same thing as the "stat" function (including setting the
3303           special "_" filehandle) but stats a symbolic link instead of the
3304           file the symbolic link points to.  If symbolic links are
3305           unimplemented on your system, a normal "stat" is done.  For much
3306           more detailed information, please see the documentation for "stat".
3307
3308           If EXPR is omitted, stats $_.
3309
3310           Portability issues: "lstat" in perlport.
3311
3312       m// The match operator.  See "Regexp Quote-Like Operators" in perlop.
3313
3314       map BLOCK LIST
3315       map EXPR,LIST
3316           Evaluates the BLOCK or EXPR for each element of LIST (locally
3317           setting $_ to each element) and composes a list of the results of
3318           each such evaluation.  Each element of LIST may produce zero, one,
3319           or more elements in the generated list, so the number of elements
3320           in the generated list may differ from that in LIST.  In scalar
3321           context, returns the total number of elements so generated.  In
3322           list context, returns the generated list.
3323
3324               my @chars = map(chr, @numbers);
3325
3326           translates a list of numbers to the corresponding characters.
3327
3328               my @squares = map { $_ * $_ } @numbers;
3329
3330           translates a list of numbers to their squared values.
3331
3332               my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3333
3334           shows that number of returned elements can differ from the number
3335           of input elements.  To omit an element, return an empty list ().
3336           This could also be achieved by writing
3337
3338               my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3339
3340           which makes the intention more clear.
3341
3342           Map always returns a list, which can be assigned to a hash such
3343           that the elements become key/value pairs.  See perldata for more
3344           details.
3345
3346               my %hash = map { get_a_key_for($_) => $_ } @array;
3347
3348           is just a funny way to write
3349
3350               my %hash;
3351               foreach (@array) {
3352                   $hash{get_a_key_for($_)} = $_;
3353               }
3354
3355           Note that $_ is an alias to the list value, so it can be used to
3356           modify the elements of the LIST.  While this is useful and
3357           supported, it can cause bizarre results if the elements of LIST are
3358           not variables.  Using a regular "foreach" loop for this purpose
3359           would be clearer in most cases.  See also "grep" for a list
3360           composed of those items of the original list for which the BLOCK or
3361           EXPR evaluates to true.
3362
3363           "{" starts both hash references and blocks, so "map { ..." could be
3364           either the start of map BLOCK LIST or map EXPR, LIST.  Because Perl
3365           doesn't look ahead for the closing "}" it has to take a guess at
3366           which it's dealing with based on what it finds just after the "{".
3367           Usually it gets it right, but if it doesn't it won't realize
3368           something is wrong until it gets to the "}" and encounters the
3369           missing (or unexpected) comma.  The syntax error will be reported
3370           close to the "}", but you'll need to change something near the "{"
3371           such as using a unary "+" or semicolon to give Perl some help:
3372
3373            my %hash = map {  "\L$_" => 1  } @array # perl guesses EXPR. wrong
3374            my %hash = map { +"\L$_" => 1  } @array # perl guesses BLOCK. right
3375            my %hash = map {; "\L$_" => 1  } @array # this also works
3376            my %hash = map { ("\L$_" => 1) } @array # as does this
3377            my %hash = map {  lc($_) => 1  } @array # and this.
3378            my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3379
3380            my %hash = map  ( lc($_), 1 ),   @array # evaluates to (1, @array)
3381
3382           or to force an anon hash constructor use "+{":
3383
3384               my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3385                                                         # comma at end
3386
3387           to get a list of anonymous hashes each with only one entry apiece.
3388
3389       mkdir FILENAME,MODE
3390       mkdir FILENAME
3391       mkdir
3392           Creates the directory specified by FILENAME, with permissions
3393           specified by MODE (as modified by "umask").  If it succeeds it
3394           returns true; otherwise it returns false and sets $! (errno).  MODE
3395           defaults to 0777 if omitted, and FILENAME defaults to $_ if
3396           omitted.
3397
3398           In general, it is better to create directories with a permissive
3399           MODE and let the user modify that with their "umask" than it is to
3400           supply a restrictive MODE and give the user no way to be more
3401           permissive.  The exceptions to this rule are when the file or
3402           directory should be kept private (mail files, for instance).  The
3403           documentation for "umask" discusses the choice of MODE in more
3404           detail.
3405
3406           Note that according to the POSIX 1003.1-1996 the FILENAME may have
3407           any number of trailing slashes.  Some operating and filesystems do
3408           not get this right, so Perl automatically removes all trailing
3409           slashes to keep everyone happy.
3410
3411           To recursively create a directory structure, look at the
3412           "make_path" function of the File::Path module.
3413
3414       msgctl ID,CMD,ARG
3415           Calls the System V IPC function msgctl(2).  You'll probably have to
3416           say
3417
3418               use IPC::SysV;
3419
3420           first to get the correct constant definitions.  If CMD is
3421           "IPC_STAT", then ARG must be a variable that will hold the returned
3422           "msqid_ds" structure.  Returns like "ioctl": the undefined value
3423           for error, "0 but true" for zero, or the actual return value
3424           otherwise.  See also "SysV IPC" in perlipc and the documentation
3425           for "IPC::SysV" and "IPC::Semaphore".
3426
3427           Portability issues: "msgctl" in perlport.
3428
3429       msgget KEY,FLAGS
3430           Calls the System V IPC function msgget(2).  Returns the message
3431           queue id, or "undef" on error.  See also "SysV IPC" in perlipc and
3432           the documentation for "IPC::SysV" and "IPC::Msg".
3433
3434           Portability issues: "msgget" in perlport.
3435
3436       msgrcv ID,VAR,SIZE,TYPE,FLAGS
3437           Calls the System V IPC function msgrcv to receive a message from
3438           message queue ID into variable VAR with a maximum message size of
3439           SIZE.  Note that when a message is received, the message type as a
3440           native long integer will be the first thing in VAR, followed by the
3441           actual message.  This packing may be opened with "unpack("l! a*")".
3442           Taints the variable.  Returns true if successful, false on error.
3443           See also "SysV IPC" in perlipc and the documentation for
3444           "IPC::SysV" and "IPC::Msg".
3445
3446           Portability issues: "msgrcv" in perlport.
3447
3448       msgsnd ID,MSG,FLAGS
3449           Calls the System V IPC function msgsnd to send the message MSG to
3450           the message queue ID.  MSG must begin with the native long integer
3451           message type, followed by the message itself.  This kind of packing
3452           can be achieved with "pack("l! a*", $type, $message)".  Returns
3453           true if successful, false on error.  See also "SysV IPC" in perlipc
3454           and the documentation for "IPC::SysV" and "IPC::Msg".
3455
3456           Portability issues: "msgsnd" in perlport.
3457
3458       my VARLIST
3459       my TYPE VARLIST
3460       my VARLIST : ATTRS
3461       my TYPE VARLIST : ATTRS
3462           A "my" declares the listed variables to be local (lexically) to the
3463           enclosing block, file, or "eval".  If more than one variable is
3464           listed, the list must be placed in parentheses.
3465
3466           Note that with a parenthesised list, "undef" can be used as a dummy
3467           placeholder, for example to skip assignment of initial values:
3468
3469               my ( undef, $min, $hour ) = localtime;
3470
3471           Redeclaring a variable in the same scope or statement will "shadow"
3472           the previous declaration, creating a new instance and preventing
3473           access to the previous one. This is usually undesired and, if
3474           warnings are enabled, will result in a warning in the "shadow"
3475           category.
3476
3477           The exact semantics and interface of TYPE and ATTRS are still
3478           evolving.  TYPE may be a bareword, a constant declared with "use
3479           constant", or "__PACKAGE__".  It is currently bound to the use of
3480           the fields pragma, and attributes are handled using the attributes
3481           pragma, or starting from Perl 5.8.0 also via the
3482           Attribute::Handlers module.  See "Private Variables via my()" in
3483           perlsub for details.
3484
3485       next LABEL
3486       next EXPR
3487       next
3488           The "next" command is like the "continue" statement in C; it starts
3489           the next iteration of the loop:
3490
3491               LINE: while (<STDIN>) {
3492                   next LINE if /^#/;  # discard comments
3493                   #...
3494               }
3495
3496           Note that if there were a "continue" block on the above, it would
3497           get executed even on discarded lines.  If LABEL is omitted, the
3498           command refers to the innermost enclosing loop.  The "next EXPR"
3499           form, available as of Perl 5.18.0, allows a label name to be
3500           computed at run time, being otherwise identical to "next LABEL".
3501
3502           "next" cannot return a value from a block that typically returns a
3503           value, such as "eval {}", "sub {}", or "do {}". It will perform its
3504           flow control behavior, which precludes any return value. It should
3505           not be used to exit a "grep" or "map" operation.
3506
3507           Note that a block by itself is semantically identical to a loop
3508           that executes once.  Thus "next" will exit such a block early.
3509
3510           See also "continue" for an illustration of how "last", "next", and
3511           "redo" work.
3512
3513           Unlike most named operators, this has the same precedence as
3514           assignment.  It is also exempt from the looks-like-a-function rule,
3515           so "next ("foo")."bar"" will cause "bar" to be part of the argument
3516           to "next".
3517
3518       no MODULE VERSION LIST
3519       no MODULE VERSION
3520       no MODULE LIST
3521       no MODULE
3522       no VERSION
3523           See the "use" function, of which "no" is the opposite.
3524
3525       oct EXPR
3526       oct Interprets EXPR as an octal string and returns the corresponding
3527           value.  An octal string consists of octal digits and, as of Perl
3528           5.33.5, an optional "0o" or "o" prefix.  Each octal digit may be
3529           preceded by a single underscore, which will be ignored.  (If EXPR
3530           happens to start off with "0x" or "x", interprets it as a hex
3531           string.  If EXPR starts off with "0b" or "b", it is interpreted as
3532           a binary string.  Leading whitespace is ignored in all three
3533           cases.)  The following will handle decimal, binary, octal, and hex
3534           in standard Perl notation:
3535
3536               $val = oct($val) if $val =~ /^0/;
3537
3538           If EXPR is omitted, uses $_.   To go the other way (produce a
3539           number in octal), use "sprintf" or "printf":
3540
3541               my $dec_perms = (stat("filename"))[2] & 07777;
3542               my $oct_perm_str = sprintf "%o", $perms;
3543
3544           The "oct" function is commonly used when a string such as 644 needs
3545           to be converted into a file mode, for example.  Although Perl
3546           automatically converts strings into numbers as needed, this
3547           automatic conversion assumes base 10.
3548
3549           Leading white space is ignored without warning, as too are any
3550           trailing non-digits, such as a decimal point ("oct" only handles
3551           non-negative integers, not negative integers or floating point).
3552
3553       open FILEHANDLE,MODE,EXPR
3554       open FILEHANDLE,MODE,EXPR,LIST
3555       open FILEHANDLE,MODE,REFERENCE
3556       open FILEHANDLE,EXPR
3557       open FILEHANDLE
3558           Associates an internal FILEHANDLE with the external file specified
3559           by EXPR. That filehandle will subsequently allow you to perform I/O
3560           operations on that file, such as reading from it or writing to it.
3561
3562           Instead of a filename, you may specify an external command (plus an
3563           optional argument list) or a scalar reference, in order to open
3564           filehandles on commands or in-memory scalars, respectively.
3565
3566           A thorough reference to "open" follows. For a gentler introduction
3567           to the basics of "open", see also the perlopentut manual page.
3568
3569           Working with files
3570               Most often, "open" gets invoked with three arguments: the
3571               required FILEHANDLE (usually an empty scalar variable),
3572               followed by MODE (usually a literal describing the I/O mode the
3573               filehandle will use), and then the filename  that the new
3574               filehandle will refer to.
3575
3576               Simple examples
3577                   Reading from a file:
3578
3579                       open(my $fh, "<", "input.txt")
3580                           or die "Can't open < input.txt: $!";
3581
3582                       # Process every line in input.txt
3583                       while (my $line = <$fh>) {
3584                           #
3585                           # ... do something interesting with $line here ...
3586                           #
3587                       }
3588
3589                   or writing to one:
3590
3591                       open(my $fh, ">", "output.txt")
3592                           or die "Can't open > output.txt: $!";
3593
3594                       print $fh "This line gets printed into output.txt.\n";
3595
3596                   For a summary of common filehandle operations such as
3597                   these, see "Files and I/O" in perlintro.
3598
3599               About filehandles
3600                   The first argument to "open", labeled FILEHANDLE in this
3601                   reference, is usually a scalar variable. (Exceptions exist,
3602                   described in "Other considerations", below.) If the call to
3603                   "open" succeeds, then the expression provided as FILEHANDLE
3604                   will get assigned an open filehandle. That filehandle
3605                   provides an internal reference to the specified external
3606                   file, conveniently stored in a Perl variable, and ready for
3607                   I/O operations such as reading and writing.
3608
3609               About modes
3610                   When calling "open" with three or more arguments, the
3611                   second argument -- labeled MODE here -- defines the open
3612                   mode. MODE is usually a literal string comprising special
3613                   characters that define the intended I/O role of the
3614                   filehandle being created: whether it's read-only, or read-
3615                   and-write, and so on.
3616
3617                   If MODE is "<", the file is opened for input (read-only).
3618                   If MODE is ">", the file is opened for output, with
3619                   existing files first being truncated ("clobbered") and
3620                   nonexisting files newly created.  If MODE is ">>", the file
3621                   is opened for appending, again being created if necessary.
3622
3623                   You can put a "+" in front of the ">" or "<" to indicate
3624                   that you want both read and write access to the file; thus
3625                   "+<" is almost always preferred for read/write updates--the
3626                   "+>" mode would clobber the file first.  You can't usually
3627                   use either read-write mode for updating textfiles, since
3628                   they have variable-length records.  See the -i switch in
3629                   perlrun for a better approach.  The file is created with
3630                   permissions of 0666 modified by the process's "umask"
3631                   value.
3632
3633                   These various prefixes correspond to the fopen(3) modes of
3634                   "r", "r+", "w", "w+", "a", and "a+".
3635
3636                   More examples of different modes in action:
3637
3638                    # Open a file for concatenation
3639                    open(my $log, ">>", "/usr/spool/news/twitlog")
3640                        or warn "Couldn't open log file; discarding input";
3641
3642                    # Open a file for reading and writing
3643                    open(my $dbase, "+<", "dbase.mine")
3644                        or die "Can't open 'dbase.mine' for update: $!";
3645
3646               Checking the return value
3647                   Open returns nonzero on success, the undefined value
3648                   otherwise.  If the "open" involved a pipe, the return value
3649                   happens to be the pid of the subprocess.
3650
3651                   When opening a file, it's seldom a good idea to continue if
3652                   the request failed, so "open" is frequently used with
3653                   "die". Even if you want your code to do something other
3654                   than "die" on a failed open, you should still always check
3655                   the return value from opening a file.
3656
3657           Specifying I/O layers in MODE
3658               You can use the three-argument form of open to specify I/O
3659               layers (sometimes referred to as "disciplines") to apply to the
3660               new filehandle. These affect how the input and output are
3661               processed (see open and PerlIO for more details).  For example:
3662
3663                   open(my $fh, "<:encoding(UTF-8)", $filename)
3664                       || die "Can't open UTF-8 encoded $filename: $!";
3665
3666               This opens the UTF8-encoded file containing Unicode characters;
3667               see perluniintro.  Note that if layers are specified in the
3668               three-argument form, then default layers stored in "${^OPEN}"
3669               (usually set by the open pragma or the switch "-CioD") are
3670               ignored.  Those layers will also be ignored if you specify a
3671               colon with no name following it.  In that case the default
3672               layer for the operating system (:raw on Unix, :crlf on Windows)
3673               is used.
3674
3675               On some systems (in general, DOS- and Windows-based systems)
3676               "binmode" is necessary when you're not working with a text
3677               file.  For the sake of portability it is a good idea always to
3678               use it when appropriate, and never to use it when it isn't
3679               appropriate.  Also, people can set their I/O to be by default
3680               UTF8-encoded Unicode, not bytes.
3681
3682           Using "undef" for temporary files
3683               As a special case the three-argument form with a read/write
3684               mode and the third argument being "undef":
3685
3686                   open(my $tmp, "+>", undef) or die ...
3687
3688               opens a filehandle to a newly created empty anonymous temporary
3689               file.  (This happens under any mode, which makes "+>" the only
3690               useful and sensible mode to use.)  You will need to "seek" to
3691               do the reading.
3692
3693           Opening a filehandle into an in-memory scalar
3694               You can open filehandles directly to Perl scalars instead of a
3695               file or other resource external to the program. To do so,
3696               provide a reference to that scalar as the third argument to
3697               "open", like so:
3698
3699                open(my $memory, ">", \$var)
3700                    or die "Can't open memory file: $!";
3701                print $memory "foo!\n";    # output will appear in $var
3702
3703               To (re)open "STDOUT" or "STDERR" as an in-memory file, close it
3704               first:
3705
3706                   close STDOUT;
3707                   open(STDOUT, ">", \$variable)
3708                       or die "Can't open STDOUT: $!";
3709
3710               The scalars for in-memory files are treated as octet strings:
3711               unless the file is being opened with truncation the scalar may
3712               not contain any code points over 0xFF.
3713
3714               Opening in-memory files can fail for a variety of reasons.  As
3715               with any other "open", check the return value for success.
3716
3717               Technical note: This feature works only when Perl is built with
3718               PerlIO -- the default, except with older (pre-5.16) Perl
3719               installations that were configured to not include it (e.g. via
3720               "Configure -Uuseperlio"). You can see whether your Perl was
3721               built with PerlIO by running "perl -V:useperlio".  If it says
3722               'define', you have PerlIO; otherwise you don't.
3723
3724               See perliol for detailed info on PerlIO.
3725
3726           Opening a filehandle into a command
3727               If MODE is "|-", then the filename is interpreted as a command
3728               to which output is to be piped, and if MODE is "-|", the
3729               filename is interpreted as a command that pipes output to us.
3730               In the two-argument (and one-argument) form, one should replace
3731               dash ("-") with the command.  See "Using open() for IPC" in
3732               perlipc for more examples of this.  (You are not allowed to
3733               "open" to a command that pipes both in and out, but see
3734               IPC::Open2, IPC::Open3, and "Bidirectional Communication with
3735               Another Process" in perlipc for alternatives.)
3736
3737                open(my $article_fh, "-|", "caesar <$article")  # decrypt
3738                                                                # article
3739                    or die "Can't start caesar: $!";
3740
3741                open(my $article_fh, "caesar <$article |")      # ditto
3742                    or die "Can't start caesar: $!";
3743
3744                open(my $out_fh, "|-", "sort >Tmp$$")    # $$ is our process id
3745                    or die "Can't start sort: $!";
3746
3747               In the form of pipe opens taking three or more arguments, if
3748               LIST is specified (extra arguments after the command name) then
3749               LIST becomes arguments to the command invoked if the platform
3750               supports it.  The meaning of "open" with more than three
3751               arguments for non-pipe modes is not yet defined, but
3752               experimental "layers" may give extra LIST arguments meaning.
3753
3754               If you open a pipe on the command "-" (that is, specify either
3755               "|-" or "-|" with the one- or two-argument forms of "open"), an
3756               implicit "fork" is done, so "open" returns twice: in the parent
3757               process it returns the pid of the child process, and in the
3758               child process it returns (a defined) 0.  Use "defined($pid)" or
3759               "//" to determine whether the open was successful.
3760
3761               For example, use either
3762
3763                  my $child_pid = open(my $from_kid, "-|")
3764                       // die "Can't fork: $!";
3765
3766               or
3767
3768                  my $child_pid = open(my $to_kid,   "|-")
3769                       // die "Can't fork: $!";
3770
3771               followed by
3772
3773                   if ($child_pid) {
3774                       # am the parent:
3775                       # either write $to_kid or else read $from_kid
3776                       ...
3777                      waitpid $child_pid, 0;
3778                   } else {
3779                       # am the child; use STDIN/STDOUT normally
3780                       ...
3781                       exit;
3782                   }
3783
3784               The filehandle behaves normally for the parent, but I/O to that
3785               filehandle is piped from/to the STDOUT/STDIN of the child
3786               process.  In the child process, the filehandle isn't
3787               opened--I/O happens from/to the new STDOUT/STDIN.  Typically
3788               this is used like the normal piped open when you want to
3789               exercise more control over just how the pipe command gets
3790               executed, such as when running setuid and you don't want to
3791               have to scan shell commands for metacharacters.
3792
3793               The following blocks are more or less equivalent:
3794
3795                   open(my $fh, "|tr '[a-z]' '[A-Z]'");
3796                   open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
3797                   open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
3798                   open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
3799
3800                   open(my $fh, "cat -n '$file'|");
3801                   open(my $fh, "-|", "cat -n '$file'");
3802                   open(my $fh, "-|") || exec "cat", "-n", $file;
3803                   open(my $fh, "-|", "cat", "-n", $file);
3804
3805               The last two examples in each block show the pipe as "list
3806               form", which is not yet supported on all platforms. (If your
3807               platform has a real "fork", such as Linux and macOS, you can
3808               use the list form; it also works on Windows with Perl 5.22 or
3809               later.) You would want to use the list form of the pipe so you
3810               can pass literal arguments to the command without risk of the
3811               shell interpreting any shell metacharacters in them. However,
3812               this also bars you from opening pipes to commands that
3813               intentionally contain shell metacharacters, such as:
3814
3815                   open(my $fh, "|cat -n | expand -4 | lpr")
3816                       || die "Can't open pipeline to lpr: $!";
3817
3818               See "Safe Pipe Opens" in perlipc for more examples of this.
3819
3820           Duping filehandles
3821               You may also, in the Bourne shell tradition, specify an EXPR
3822               beginning with ">&", in which case the rest of the string is
3823               interpreted as the name of a filehandle (or file descriptor, if
3824               numeric) to be duped (as in dup(2)) and opened.  You may use
3825               "&" after ">", ">>", "<", "+>", "+>>", and "+<".  The mode you
3826               specify should match the mode of the original filehandle.
3827               (Duping a filehandle does not take into account any existing
3828               contents of IO buffers.)  If you use the three-argument form,
3829               then you can pass either a number, the name of a filehandle, or
3830               the normal "reference to a glob".
3831
3832               Here is a script that saves, redirects, and restores "STDOUT"
3833               and "STDERR" using various methods:
3834
3835                   #!/usr/bin/perl
3836                   open(my $oldout, ">&STDOUT")
3837                       or die "Can't dup STDOUT: $!";
3838                   open(OLDERR,     ">&", \*STDERR)
3839                       or die "Can't dup STDERR: $!";
3840
3841                   open(STDOUT, '>', "foo.out")
3842                       or die "Can't redirect STDOUT: $!";
3843                   open(STDERR, ">&STDOUT")
3844                       or die "Can't dup STDOUT: $!";
3845
3846                   select STDERR; $| = 1;  # make unbuffered
3847                   select STDOUT; $| = 1;  # make unbuffered
3848
3849                   print STDOUT "stdout 1\n";  # this works for
3850                   print STDERR "stderr 1\n";  # subprocesses too
3851
3852                   open(STDOUT, ">&", $oldout)
3853                       or die "Can't dup \$oldout: $!";
3854                   open(STDERR, ">&OLDERR")
3855                       or die "Can't dup OLDERR: $!";
3856
3857                   print STDOUT "stdout 2\n";
3858                   print STDERR "stderr 2\n";
3859
3860               If you specify '<&=X', where "X" is a file descriptor number or
3861               a filehandle, then Perl will do an equivalent of C's fdopen(3)
3862               of that file descriptor (and not call dup(2)); this is more
3863               parsimonious of file descriptors.  For example:
3864
3865                   # open for input, reusing the fileno of $fd
3866                   open(my $fh, "<&=", $fd)
3867
3868               or
3869
3870                   open(my $fh, "<&=$fd")
3871
3872               or
3873
3874                   # open for append, using the fileno of $oldfh
3875                   open(my $fh, ">>&=", $oldfh)
3876
3877               Being parsimonious on filehandles is also useful (besides being
3878               parsimonious) for example when something is dependent on file
3879               descriptors, like for example locking using "flock".  If you do
3880               just "open(my $A, ">>&", $B)", the filehandle $A will not have
3881               the same file descriptor as $B, and therefore "flock($A)" will
3882               not "flock($B)" nor vice versa.  But with "open(my $A, ">>&=",
3883               $B)", the filehandles will share the same underlying system
3884               file descriptor.
3885
3886               Note that under Perls older than 5.8.0, Perl uses the standard
3887               C library's' fdopen(3) to implement the "=" functionality.  On
3888               many Unix systems, fdopen(3) fails when file descriptors exceed
3889               a certain value, typically 255.  For Perls 5.8.0 and later,
3890               PerlIO is (most often) the default.
3891
3892           Legacy usage
3893               This section describes ways to call "open" outside of best
3894               practices; you may encounter these uses in older code. Perl
3895               does not consider their use deprecated, exactly, but neither is
3896               it recommended in new code, for the sake of clarity and
3897               readability.
3898
3899               Specifying mode and filename as a single argument
3900                   In the one- and two-argument forms of the call, the mode
3901                   and filename should be concatenated (in that order),
3902                   preferably separated by white space.  You can--but
3903                   shouldn't--omit the mode in these forms when that mode is
3904                   "<".  It is safe to use the two-argument form of "open" if
3905                   the filename argument is a known literal.
3906
3907                    open(my $dbase, "+<dbase.mine")          # ditto
3908                        or die "Can't open 'dbase.mine' for update: $!";
3909
3910                   In the two-argument (and one-argument) form, opening "<-"
3911                   or "-" opens STDIN and opening ">-" opens STDOUT.
3912
3913                   New code should favor the three-argument form of "open"
3914                   over this older form. Declaring the mode and the filename
3915                   as two distinct arguments avoids any confusion between the
3916                   two.
3917
3918               Calling "open" with one argument via global variables
3919                   As a shortcut, a one-argument call takes the filename from
3920                   the global scalar variable of the same name as the
3921                   filehandle:
3922
3923                       $ARTICLE = 100;
3924                       open(ARTICLE)
3925                           or die "Can't find article $ARTICLE: $!\n";
3926
3927                   Here $ARTICLE must be a global (package) scalar variable -
3928                   not one declared with "my" or "state".
3929
3930               Assigning a filehandle to a bareword
3931                   An older style is to use a bareword as the filehandle, as
3932
3933                       open(FH, "<", "input.txt")
3934                          or die "Can't open < input.txt: $!";
3935
3936                   Then you can use "FH" as the filehandle, in "close FH" and
3937                   "<FH>" and so on.  Note that it's a global variable, so
3938                   this form is not recommended when dealing with filehandles
3939                   other than Perl's built-in ones (e.g. STDOUT and STDIN).
3940
3941           Other considerations
3942               Automatic filehandle closure
3943                   The filehandle will be closed when its reference count
3944                   reaches zero. If it is a lexically scoped variable declared
3945                   with "my", that usually means the end of the enclosing
3946                   scope.  However, this automatic close does not check for
3947                   errors, so it is better to explicitly close filehandles,
3948                   especially those used for writing:
3949
3950                       close($handle)
3951                          || warn "close failed: $!";
3952
3953               Automatic pipe flushing
3954                   Perl will attempt to flush all files opened for output
3955                   before any operation that may do a fork, but this may not
3956                   be supported on some platforms (see perlport).  To be safe,
3957                   you may need to set $| ($AUTOFLUSH in English) or call the
3958                   "autoflush" method of "IO::Handle" on any open handles.
3959
3960                   On systems that support a close-on-exec flag on files, the
3961                   flag will be set for the newly opened file descriptor as
3962                   determined by the value of $^F.  See "$^F" in perlvar.
3963
3964                   Closing any piped filehandle causes the parent process to
3965                   wait for the child to finish, then returns the status value
3966                   in $? and "${^CHILD_ERROR_NATIVE}".
3967
3968               Direct versus by-reference assignment of filehandles
3969                   If FILEHANDLE -- the first argument in a call to "open" --
3970                   is an undefined scalar variable (or array or hash element),
3971                   a new filehandle is autovivified, meaning that the variable
3972                   is assigned a reference to a newly allocated anonymous
3973                   filehandle.  Otherwise if FILEHANDLE is an expression, its
3974                   value is the real filehandle.  (This is considered a
3975                   symbolic reference, so "use strict "refs"" should not be in
3976                   effect.)
3977
3978               Whitespace and special characters in the filename argument
3979                   The filename passed to the one- and two-argument forms of
3980                   "open" will have leading and trailing whitespace deleted
3981                   and normal redirection characters honored.  This property,
3982                   known as "magic open", can often be used to good effect.  A
3983                   user could specify a filename of "rsh cat file |", or you
3984                   could change certain filenames as needed:
3985
3986                       $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3987                       open(my $fh, $filename)
3988                           or die "Can't open $filename: $!";
3989
3990                   Use the three-argument form to open a file with arbitrary
3991                   weird characters in it,
3992
3993                       open(my $fh, "<", $file)
3994                           || die "Can't open $file: $!";
3995
3996                   otherwise it's necessary to protect any leading and
3997                   trailing whitespace:
3998
3999                       $file =~ s#^(\s)#./$1#;
4000                       open(my $fh, "< $file\0")
4001                           || die "Can't open $file: $!";
4002
4003                   (this may not work on some bizarre filesystems).  One
4004                   should conscientiously choose between the magic and three-
4005                   argument form of "open":
4006
4007                       open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
4008
4009                   will allow the user to specify an argument of the form "rsh
4010                   cat file |", but will not work on a filename that happens
4011                   to have a trailing space, while
4012
4013                       open(my $in, "<", $ARGV[0])
4014                           || die "Can't open $ARGV[0]: $!";
4015
4016                   will have exactly the opposite restrictions. (However, some
4017                   shells support the syntax "perl your_program.pl <( rsh cat
4018                   file )", which produces a filename that can be opened
4019                   normally.)
4020
4021               Invoking C-style "open"
4022                   If you want a "real" C open(2), then you should use the
4023                   "sysopen" function, which involves no such magic (but uses
4024                   different filemodes than Perl "open", which corresponds to
4025                   C fopen(3)).  This is another way to protect your filenames
4026                   from interpretation.  For example:
4027
4028                       use IO::Handle;
4029                       sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
4030                           or die "Can't open $path: $!";
4031                       $fh->autoflush(1);
4032                       print $fh "stuff $$\n";
4033                       seek($fh, 0, 0);
4034                       print "File contains: ", readline($fh);
4035
4036                   See "seek" for some details about mixing reading and
4037                   writing.
4038
4039               Portability issues
4040                   See "open" in perlport.
4041
4042       opendir DIRHANDLE,EXPR
4043           Opens a directory named EXPR for processing by "readdir",
4044           "telldir", "seekdir", "rewinddir", and "closedir".  Returns true if
4045           successful.  DIRHANDLE may be an expression whose value can be used
4046           as an indirect dirhandle, usually the real dirhandle name.  If
4047           DIRHANDLE is an undefined scalar variable (or array or hash
4048           element), the variable is assigned a reference to a new anonymous
4049           dirhandle; that is, it's autovivified.  Dirhandles are the same
4050           objects as filehandles; an I/O object can only be open as one of
4051           these handle types at once.
4052
4053           See the example at "readdir".
4054
4055       ord EXPR
4056       ord Returns the numeric value of the first character of EXPR.  If EXPR
4057           is an empty string, returns 0.  If EXPR is omitted, uses $_.  (Note
4058           character, not byte.)
4059
4060           For the reverse, see "chr".  See perlunicode for more about
4061           Unicode.
4062
4063       our VARLIST
4064       our TYPE VARLIST
4065       our VARLIST : ATTRS
4066       our TYPE VARLIST : ATTRS
4067           "our" makes a lexical alias to a package (i.e. global) variable of
4068           the same name in the current package for use within the current
4069           lexical scope.
4070
4071           "our" has the same scoping rules as "my" or "state", meaning that
4072           it is only valid within a lexical scope.  Unlike "my" and "state",
4073           which both declare new (lexical) variables, "our" only creates an
4074           alias to an existing variable: a package variable of the same name.
4075
4076           This means that when "use strict 'vars'" is in effect, "our" lets
4077           you use a package variable without qualifying it with the package
4078           name, but only within the lexical scope of the "our" declaration.
4079           This applies immediately--even within the same statement.
4080
4081               package Foo;
4082               use strict;
4083
4084               $Foo::foo = 23;
4085
4086               {
4087                   our $foo;   # alias to $Foo::foo
4088                   print $foo; # prints 23
4089               }
4090
4091               print $Foo::foo; # prints 23
4092
4093               print $foo; # ERROR: requires explicit package name
4094
4095           This works even if the package variable has not been used before,
4096           as package variables spring into existence when first used.
4097
4098               package Foo;
4099               use strict;
4100
4101               our $foo = 23;   # just like $Foo::foo = 23
4102
4103               print $Foo::foo; # prints 23
4104
4105           Because the variable becomes legal immediately under "use strict
4106           'vars'", so long as there is no variable with that name is already
4107           in scope, you can then reference the package variable again even
4108           within the same statement.
4109
4110               package Foo;
4111               use strict;
4112
4113               my  $foo = $foo; # error, undeclared $foo on right-hand side
4114               our $foo = $foo; # no errors
4115
4116           If more than one variable is listed, the list must be placed in
4117           parentheses.
4118
4119               our($bar, $baz);
4120
4121           An "our" declaration declares an alias for a package variable that
4122           will be visible across its entire lexical scope, even across
4123           package boundaries.  The package in which the variable is entered
4124           is determined at the point of the declaration, not at the point of
4125           use.  This means the following behavior holds:
4126
4127               package Foo;
4128               our $bar;      # declares $Foo::bar for rest of lexical scope
4129               $bar = 20;
4130
4131               package Bar;
4132               print $bar;    # prints 20, as it refers to $Foo::bar
4133
4134           Multiple "our" declarations with the same name in the same lexical
4135           scope are allowed if they are in different packages.  If they
4136           happen to be in the same package, Perl will emit warnings if you
4137           have asked for them, just like multiple "my" declarations.  Unlike
4138           a second "my" declaration, which will bind the name to a fresh
4139           variable, a second "our" declaration in the same package, in the
4140           same scope, is merely redundant.
4141
4142               use warnings;
4143               package Foo;
4144               our $bar;      # declares $Foo::bar for rest of lexical scope
4145               $bar = 20;
4146
4147               package Bar;
4148               our $bar = 30; # declares $Bar::bar for rest of lexical scope
4149               print $bar;    # prints 30
4150
4151               our $bar;      # emits warning but has no other effect
4152               print $bar;    # still prints 30
4153
4154           An "our" declaration may also have a list of attributes associated
4155           with it.
4156
4157           The exact semantics and interface of TYPE and ATTRS are still
4158           evolving.  TYPE is currently bound to the use of the fields pragma,
4159           and attributes are handled using the attributes pragma, or,
4160           starting from Perl 5.8.0, also via the Attribute::Handlers module.
4161           See "Private Variables via my()" in perlsub for details.
4162
4163           Note that with a parenthesised list, "undef" can be used as a dummy
4164           placeholder, for example to skip assignment of initial values:
4165
4166               our ( undef, $min, $hour ) = localtime;
4167
4168           "our" differs from "use vars", which allows use of an unqualified
4169           name only within the affected package, but across scopes.
4170
4171       pack TEMPLATE,LIST
4172           Takes a LIST of values and converts it into a string using the
4173           rules given by the TEMPLATE.  The resulting string is the
4174           concatenation of the converted values.  Typically, each converted
4175           value looks like its machine-level representation.  For example, on
4176           32-bit machines an integer may be represented by a sequence of 4
4177           bytes, which  will in Perl be presented as a string that's 4
4178           characters long.
4179
4180           See perlpacktut for an introduction to this function.
4181
4182           The TEMPLATE is a sequence of characters that give the order and
4183           type of values, as follows:
4184
4185               a  A string with arbitrary binary data, will be null padded.
4186               A  A text (ASCII) string, will be space padded.
4187               Z  A null-terminated (ASCIZ) string, will be null padded.
4188
4189               b  A bit string (ascending bit order inside each byte,
4190                  like vec()).
4191               B  A bit string (descending bit order inside each byte).
4192               h  A hex string (low nybble first).
4193               H  A hex string (high nybble first).
4194
4195               c  A signed char (8-bit) value.
4196               C  An unsigned char (octet) value.
4197               W  An unsigned char value (can be greater than 255).
4198
4199               s  A signed short (16-bit) value.
4200               S  An unsigned short value.
4201
4202               l  A signed long (32-bit) value.
4203               L  An unsigned long value.
4204
4205               q  A signed quad (64-bit) value.
4206               Q  An unsigned quad value.
4207                    (Quads are available only if your system supports 64-bit
4208                     integer values _and_ if Perl has been compiled to support
4209                     those.  Raises an exception otherwise.)
4210
4211               i  A signed integer value.
4212               I  An unsigned integer value.
4213                    (This 'integer' is _at_least_ 32 bits wide.  Its exact
4214                     size depends on what a local C compiler calls 'int'.)
4215
4216               n  An unsigned short (16-bit) in "network" (big-endian) order.
4217               N  An unsigned long (32-bit) in "network" (big-endian) order.
4218               v  An unsigned short (16-bit) in "VAX" (little-endian) order.
4219               V  An unsigned long (32-bit) in "VAX" (little-endian) order.
4220
4221               j  A Perl internal signed integer value (IV).
4222               J  A Perl internal unsigned integer value (UV).
4223
4224               f  A single-precision float in native format.
4225               d  A double-precision float in native format.
4226
4227               F  A Perl internal floating-point value (NV) in native format
4228               D  A float of long-double precision in native format.
4229                    (Long doubles are available only if your system supports
4230                     long double values. Raises an exception otherwise.
4231                     Note that there are different long double formats.)
4232
4233               p  A pointer to a null-terminated string.
4234               P  A pointer to a structure (fixed-length string).
4235
4236               u  A uuencoded string.
4237               U  A Unicode character number.  Encodes to a character in char-
4238                  acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4239                  byte mode.
4240
4241               w  A BER compressed integer (not an ASN.1 BER, see perlpacktut
4242                  for details).  Its bytes represent an unsigned integer in
4243                  base 128, most significant digit first, with as few digits
4244                  as possible.  Bit eight (the high bit) is set on each byte
4245                  except the last.
4246
4247               x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
4248               X  Back up a byte.
4249               @  Null-fill or truncate to absolute position, counted from the
4250                  start of the innermost ()-group.
4251               .  Null-fill or truncate to absolute position specified by
4252                  the value.
4253               (  Start of a ()-group.
4254
4255           One or more modifiers below may optionally follow certain letters
4256           in the TEMPLATE (the second column lists letters for which the
4257           modifier is valid):
4258
4259               !   sSlLiI     Forces native (short, long, int) sizes instead
4260                              of fixed (16-/32-bit) sizes.
4261
4262               !   xX         Make x and X act as alignment commands.
4263
4264               !   nNvV       Treat integers as signed instead of unsigned.
4265
4266               !   @.         Specify position as byte offset in the internal
4267                              representation of the packed string.  Efficient
4268                              but dangerous.
4269
4270               >   sSiIlLqQ   Force big-endian byte-order on the type.
4271                   jJfFdDpP   (The "big end" touches the construct.)
4272
4273               <   sSiIlLqQ   Force little-endian byte-order on the type.
4274                   jJfFdDpP   (The "little end" touches the construct.)
4275
4276           The ">" and "<" modifiers can also be used on "()" groups to force
4277           a particular byte-order on all components in that group, including
4278           all its subgroups.
4279
4280           The following rules apply:
4281
4282           •   Each letter may optionally be followed by a number indicating
4283               the repeat count.  A numeric repeat count may optionally be
4284               enclosed in brackets, as in "pack("C[80]", @arr)".  The repeat
4285               count gobbles that many values from the LIST when used with all
4286               format types other than "a", "A", "Z", "b", "B", "h", "H", "@",
4287               ".", "x", "X", and "P", where it means something else,
4288               described below.  Supplying a "*" for the repeat count instead
4289               of a number means to use however many items are left, except
4290               for:
4291
4292               •   "@", "x", and "X", where it is equivalent to 0.
4293
4294               •   <.>, where it means relative to the start of the string.
4295
4296               •   "u", where it is equivalent to 1 (or 45, which here is
4297                   equivalent).
4298
4299               One can replace a numeric repeat count with a template letter
4300               enclosed in brackets to use the packed byte length of the
4301               bracketed template for the repeat count.
4302
4303               For example, the template "x[L]" skips as many bytes as in a
4304               packed long, and the template "$t X[$t] $t" unpacks twice
4305               whatever $t (when variable-expanded) unpacks.  If the template
4306               in brackets contains alignment commands (such as "x![d]"), its
4307               packed length is calculated as if the start of the template had
4308               the maximal possible alignment.
4309
4310               When used with "Z", a "*" as the repeat count is guaranteed to
4311               add a trailing null byte, so the resulting string is always one
4312               byte longer than the byte length of the item itself.
4313
4314               When used with "@", the repeat count represents an offset from
4315               the start of the innermost "()" group.
4316
4317               When used with ".", the repeat count determines the starting
4318               position to calculate the value offset as follows:
4319
4320               •   If the repeat count is 0, it's relative to the current
4321                   position.
4322
4323               •   If the repeat count is "*", the offset is relative to the
4324                   start of the packed string.
4325
4326               •   And if it's an integer n, the offset is relative to the
4327                   start of the nth innermost "( )" group, or to the start of
4328                   the string if n is bigger then the group level.
4329
4330               The repeat count for "u" is interpreted as the maximal number
4331               of bytes to encode per line of output, with 0, 1 and 2 replaced
4332               by 45.  The repeat count should not be more than 65.
4333
4334           •   The "a", "A", and "Z" types gobble just one value, but pack it
4335               as a string of length count, padding with nulls or spaces as
4336               needed.  When unpacking, "A" strips trailing whitespace and
4337               nulls, "Z" strips everything after the first null, and "a"
4338               returns data with no stripping at all.
4339
4340               If the value to pack is too long, the result is truncated.  If
4341               it's too long and an explicit count is provided, "Z" packs only
4342               "$count-1" bytes, followed by a null byte.  Thus "Z" always
4343               packs a trailing null, except when the count is 0.
4344
4345           •   Likewise, the "b" and "B" formats pack a string that's that
4346               many bits long.  Each such format generates 1 bit of the
4347               result.  These are typically followed by a repeat count like
4348               "B8" or "B64".
4349
4350               Each result bit is based on the least-significant bit of the
4351               corresponding input character, i.e., on "ord($char)%2".  In
4352               particular, characters "0" and "1" generate bits 0 and 1, as do
4353               characters "\000" and "\001".
4354
4355               Starting from the beginning of the input string, each 8-tuple
4356               of characters is converted to 1 character of output.  With
4357               format "b", the first character of the 8-tuple determines the
4358               least-significant bit of a character; with format "B", it
4359               determines the most-significant bit of a character.
4360
4361               If the length of the input string is not evenly divisible by 8,
4362               the remainder is packed as if the input string were padded by
4363               null characters at the end.  Similarly during unpacking,
4364               "extra" bits are ignored.
4365
4366               If the input string is longer than needed, remaining characters
4367               are ignored.
4368
4369               A "*" for the repeat count uses all characters of the input
4370               field.  On unpacking, bits are converted to a string of 0s and
4371               1s.
4372
4373           •   The "h" and "H" formats pack a string that many nybbles (4-bit
4374               groups, representable as hexadecimal digits, "0".."9" "a".."f")
4375               long.
4376
4377               For each such format, "pack" generates 4 bits of result.  With
4378               non-alphabetical characters, the result is based on the 4
4379               least-significant bits of the input character, i.e., on
4380               "ord($char)%16".  In particular, characters "0" and "1"
4381               generate nybbles 0 and 1, as do bytes "\000" and "\001".  For
4382               characters "a".."f" and "A".."F", the result is compatible with
4383               the usual hexadecimal digits, so that "a" and "A" both generate
4384               the nybble "0xA==10".  Use only these specific hex characters
4385               with this format.
4386
4387               Starting from the beginning of the template to "pack", each
4388               pair of characters is converted to 1 character of output.  With
4389               format "h", the first character of the pair determines the
4390               least-significant nybble of the output character; with format
4391               "H", it determines the most-significant nybble.
4392
4393               If the length of the input string is not even, it behaves as if
4394               padded by a null character at the end.  Similarly, "extra"
4395               nybbles are ignored during unpacking.
4396
4397               If the input string is longer than needed, extra characters are
4398               ignored.
4399
4400               A "*" for the repeat count uses all characters of the input
4401               field.  For "unpack", nybbles are converted to a string of
4402               hexadecimal digits.
4403
4404           •   The "p" format packs a pointer to a null-terminated string.
4405               You are responsible for ensuring that the string is not a
4406               temporary value, as that could potentially get deallocated
4407               before you got around to using the packed result.  The "P"
4408               format packs a pointer to a structure of the size indicated by
4409               the length.  A null pointer is created if the corresponding
4410               value for "p" or "P" is "undef"; similarly with "unpack", where
4411               a null pointer unpacks into "undef".
4412
4413               If your system has a strange pointer size--meaning a pointer is
4414               neither as big as an int nor as big as a long--it may not be
4415               possible to pack or unpack pointers in big- or little-endian
4416               byte order.  Attempting to do so raises an exception.
4417
4418           •   The "/" template character allows packing and unpacking of a
4419               sequence of items where the packed structure contains a packed
4420               item count followed by the packed items themselves.  This is
4421               useful when the structure you're unpacking has encoded the
4422               sizes or repeat counts for some of its fields within the
4423               structure itself as separate fields.
4424
4425               For "pack", you write length-item"/"sequence-item, and the
4426               length-item describes how the length value is packed.  Formats
4427               likely to be of most use are integer-packing ones like "n" for
4428               Java strings, "w" for ASN.1 or SNMP, and "N" for Sun XDR.
4429
4430               For "pack", sequence-item may have a repeat count, in which
4431               case the minimum of that and the number of available items is
4432               used as the argument for length-item.  If it has no repeat
4433               count or uses a '*', the number of available items is used.
4434
4435               For "unpack", an internal stack of integer arguments unpacked
4436               so far is used.  You write "/"sequence-item and the repeat
4437               count is obtained by popping off the last element from the
4438               stack.  The sequence-item must not have a repeat count.
4439
4440               If sequence-item refers to a string type ("A", "a", or "Z"),
4441               the length-item is the string length, not the number of
4442               strings.  With an explicit repeat count for pack, the packed
4443               string is adjusted to that length.  For example:
4444
4445                This code:                             gives this result:
4446
4447                unpack("W/a", "\004Gurusamy")          ("Guru")
4448                unpack("a3/A A*", "007 Bond  J ")      (" Bond", "J")
4449                unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4450
4451                pack("n/a* w/a","hello,","world")     "\000\006hello,\005world"
4452                pack("a/W2", ord("a") .. ord("z"))    "2ab"
4453
4454               The length-item is not returned explicitly from "unpack".
4455
4456               Supplying a count to the length-item format letter is only
4457               useful with "A", "a", or "Z".  Packing with a length-item of
4458               "a" or "Z" may introduce "\000" characters, which Perl does not
4459               regard as legal in numeric strings.
4460
4461           •   The integer types "s", "S", "l", and "L" may be followed by a
4462               "!" modifier to specify native shorts or longs.  As shown in
4463               the example above, a bare "l" means exactly 32 bits, although
4464               the native "long" as seen by the local C compiler may be
4465               larger.  This is mainly an issue on 64-bit platforms.  You can
4466               see whether using "!" makes any difference this way:
4467
4468                   printf "format s is %d, s! is %d\n",
4469                       length pack("s"), length pack("s!");
4470
4471                   printf "format l is %d, l! is %d\n",
4472                       length pack("l"), length pack("l!");
4473
4474               "i!" and "I!" are also allowed, but only for completeness'
4475               sake: they are identical to "i" and "I".
4476
4477               The actual sizes (in bytes) of native shorts, ints, longs, and
4478               long longs on the platform where Perl was built are also
4479               available from the command line:
4480
4481                   $ perl -V:{short,int,long{,long}}size
4482                   shortsize='2';
4483                   intsize='4';
4484                   longsize='4';
4485                   longlongsize='8';
4486
4487               or programmatically via the "Config" module:
4488
4489                      use Config;
4490                      print $Config{shortsize},    "\n";
4491                      print $Config{intsize},      "\n";
4492                      print $Config{longsize},     "\n";
4493                      print $Config{longlongsize}, "\n";
4494
4495               $Config{longlongsize} is undefined on systems without long long
4496               support.
4497
4498           •   The integer formats "s", "S", "i", "I", "l", "L", "j", and "J"
4499               are inherently non-portable between processors and operating
4500               systems because they obey native byteorder and endianness.  For
4501               example, a 4-byte integer 0x12345678 (305419896 decimal) would
4502               be ordered natively (arranged in and handled by the CPU
4503               registers) into bytes as
4504
4505                   0x12 0x34 0x56 0x78  # big-endian
4506                   0x78 0x56 0x34 0x12  # little-endian
4507
4508               Basically, Intel and VAX CPUs are little-endian, while
4509               everybody else, including Motorola m68k/88k, PPC, Sparc, HP PA,
4510               Power, and Cray, are big-endian.  Alpha and MIPS can be either:
4511               Digital/Compaq uses (well, used) them in little-endian mode,
4512               but SGI/Cray uses them in big-endian mode.
4513
4514               The names big-endian and little-endian are comic references to
4515               the egg-eating habits of the little-endian Lilliputians and the
4516               big-endian Blefuscudians from the classic Jonathan Swift
4517               satire, Gulliver's Travels.  This entered computer lingo via
4518               the paper "On Holy Wars and a Plea for Peace" by Danny Cohen,
4519               USC/ISI IEN 137, April 1, 1980.
4520
4521               Some systems may have even weirder byte orders such as
4522
4523                  0x56 0x78 0x12 0x34
4524                  0x34 0x12 0x78 0x56
4525
4526               These are called mid-endian, middle-endian, mixed-endian, or
4527               just weird.
4528
4529               You can determine your system endianness with this incantation:
4530
4531                  printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4532
4533               The byteorder on the platform where Perl was built is also
4534               available via Config:
4535
4536                   use Config;
4537                   print "$Config{byteorder}\n";
4538
4539               or from the command line:
4540
4541                   $ perl -V:byteorder
4542
4543               Byteorders "1234" and "12345678" are little-endian; "4321" and
4544               "87654321" are big-endian.  Systems with multiarchitecture
4545               binaries will have "ffff", signifying that static information
4546               doesn't work, one must use runtime probing.
4547
4548               For portably packed integers, either use the formats "n", "N",
4549               "v", and "V" or else use the ">" and "<" modifiers described
4550               immediately below.  See also perlport.
4551
4552           •   Also floating point numbers have endianness.  Usually (but not
4553               always) this agrees with the integer endianness.  Even though
4554               most platforms these days use the IEEE 754 binary format, there
4555               are differences, especially if the long doubles are involved.
4556               You can see the "Config" variables "doublekind" and
4557               "longdblkind" (also "doublesize", "longdblsize"): the "kind"
4558               values are enums, unlike "byteorder".
4559
4560               Portability-wise the best option is probably to keep to the
4561               IEEE 754 64-bit doubles, and of agreed-upon endianness.
4562               Another possibility is the "%a") format of "printf".
4563
4564           •   Starting with Perl 5.10.0, integer and floating-point formats,
4565               along with the "p" and "P" formats and "()" groups, may all be
4566               followed by the ">" or "<" endianness modifiers to respectively
4567               enforce big- or little-endian byte-order.  These modifiers are
4568               especially useful given how "n", "N", "v", and "V" don't cover
4569               signed integers, 64-bit integers, or floating-point values.
4570
4571               Here are some concerns to keep in mind when using an endianness
4572               modifier:
4573
4574               •   Exchanging signed integers between different platforms
4575                   works only when all platforms store them in the same
4576                   format.  Most platforms store signed integers in two's-
4577                   complement notation, so usually this is not an issue.
4578
4579               •   The ">" or "<" modifiers can only be used on floating-point
4580                   formats on big- or little-endian machines.  Otherwise,
4581                   attempting to use them raises an exception.
4582
4583               •   Forcing big- or little-endian byte-order on floating-point
4584                   values for data exchange can work only if all platforms use
4585                   the same binary representation such as IEEE floating-point.
4586                   Even if all platforms are using IEEE, there may still be
4587                   subtle differences.  Being able to use ">" or "<" on
4588                   floating-point values can be useful, but also dangerous if
4589                   you don't know exactly what you're doing.  It is not a
4590                   general way to portably store floating-point values.
4591
4592               •   When using ">" or "<" on a "()" group, this affects all
4593                   types inside the group that accept byte-order modifiers,
4594                   including all subgroups.  It is silently ignored for all
4595                   other types.  You are not allowed to override the byte-
4596                   order within a group that already has a byte-order modifier
4597                   suffix.
4598
4599           •   Real numbers (floats and doubles) are in native machine format
4600               only.  Due to the multiplicity of floating-point formats and
4601               the lack of a standard "network" representation for them, no
4602               facility for interchange has been made.  This means that packed
4603               floating-point data written on one machine may not be readable
4604               on another, even if both use IEEE floating-point arithmetic
4605               (because the endianness of the memory representation is not
4606               part of the IEEE spec).  See also perlport.
4607
4608               If you know exactly what you're doing, you can use the ">" or
4609               "<" modifiers to force big- or little-endian byte-order on
4610               floating-point values.
4611
4612               Because Perl uses doubles (or long doubles, if configured)
4613               internally for all numeric calculation, converting from double
4614               into float and thence to double again loses precision, so
4615               "unpack("f", pack("f", $foo)") will not in general equal $foo.
4616
4617           •   Pack and unpack can operate in two modes: character mode ("C0"
4618               mode) where the packed string is processed per character, and
4619               UTF-8 byte mode ("U0" mode) where the packed string is
4620               processed in its UTF-8-encoded Unicode form on a byte-by-byte
4621               basis.  Character mode is the default unless the format string
4622               starts with "U".  You can always switch mode mid-format with an
4623               explicit "C0" or "U0" in the format.  This mode remains in
4624               effect until the next mode change, or until the end of the "()"
4625               group it (directly) applies to.
4626
4627               Using "C0" to get Unicode characters while using "U0" to get
4628               non-Unicode bytes is not necessarily obvious.   Probably only
4629               the first of these is what you want:
4630
4631                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4632                     perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4633                   03B1.03C9
4634                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4635                     perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4636                   CE.B1.CF.89
4637                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4638                     perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4639                   CE.B1.CF.89
4640                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4641                     perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4642                   C3.8E.C2.B1.C3.8F.C2.89
4643
4644               Those examples also illustrate that you should not try to use
4645               "pack"/"unpack" as a substitute for the Encode module.
4646
4647           •   You must yourself do any alignment or padding by inserting, for
4648               example, enough "x"es while packing.  There is no way for
4649               "pack" and "unpack" to know where characters are going to or
4650               coming from, so they handle their output and input as flat
4651               sequences of characters.
4652
4653           •   A "()" group is a sub-TEMPLATE enclosed in parentheses.  A
4654               group may take a repeat count either as postfix, or for
4655               "unpack", also via the "/" template character.  Within each
4656               repetition of a group, positioning with "@" starts over at 0.
4657               Therefore, the result of
4658
4659                   pack("@1A((@2A)@3A)", qw[X Y Z])
4660
4661               is the string "\0X\0\0YZ".
4662
4663           •   "x" and "X" accept the "!" modifier to act as alignment
4664               commands: they jump forward or back to the closest position
4665               aligned at a multiple of "count" characters.  For example, to
4666               "pack" or "unpack" a C structure like
4667
4668                   struct {
4669                       char   c;    /* one signed, 8-bit character */
4670                       double d;
4671                       char   cc[2];
4672                   }
4673
4674               one may need to use the template "c x![d] d c[2]".  This
4675               assumes that doubles must be aligned to the size of double.
4676
4677               For alignment commands, a "count" of 0 is equivalent to a
4678               "count" of 1; both are no-ops.
4679
4680           •   "n", "N", "v" and "V" accept the "!" modifier to represent
4681               signed 16-/32-bit integers in big-/little-endian order.  This
4682               is portable only when all platforms sharing packed data use the
4683               same binary representation for signed integers; for example,
4684               when all platforms use two's-complement representation.
4685
4686           •   Comments can be embedded in a TEMPLATE using "#" through the
4687               end of line.  White space can separate pack codes from each
4688               other, but modifiers and repeat counts must follow immediately.
4689               Breaking complex templates into individual line-by-line
4690               components, suitably annotated, can do as much to improve
4691               legibility and maintainability of pack/unpack formats as "/x"
4692               can for complicated pattern matches.
4693
4694           •   If TEMPLATE requires more arguments than "pack" is given,
4695               "pack" assumes additional "" arguments.  If TEMPLATE requires
4696               fewer arguments than given, extra arguments are ignored.
4697
4698           •   Attempting to pack the special floating point values "Inf" and
4699               "NaN" (infinity, also in negative, and not-a-number) into
4700               packed integer values (like "L") is a fatal error.  The reason
4701               for this is that there simply isn't any sensible mapping for
4702               these special values into integers.
4703
4704           Examples:
4705
4706               $foo = pack("WWWW",65,66,67,68);
4707               # foo eq "ABCD"
4708               $foo = pack("W4",65,66,67,68);
4709               # same thing
4710               $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4711               # same thing with Unicode circled letters.
4712               $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4713               # same thing with Unicode circled letters.  You don't get the
4714               # UTF-8 bytes because the U at the start of the format caused
4715               # a switch to U0-mode, so the UTF-8 bytes get joined into
4716               # characters
4717               $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4718               # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4719               # This is the UTF-8 encoding of the string in the
4720               # previous example
4721
4722               $foo = pack("ccxxcc",65,66,67,68);
4723               # foo eq "AB\0\0CD"
4724
4725               # NOTE: The examples above featuring "W" and "c" are true
4726               # only on ASCII and ASCII-derived systems such as ISO Latin 1
4727               # and UTF-8.  On EBCDIC systems, the first example would be
4728               #      $foo = pack("WWWW",193,194,195,196);
4729
4730               $foo = pack("s2",1,2);
4731               # "\001\000\002\000" on little-endian
4732               # "\000\001\000\002" on big-endian
4733
4734               $foo = pack("a4","abcd","x","y","z");
4735               # "abcd"
4736
4737               $foo = pack("aaaa","abcd","x","y","z");
4738               # "axyz"
4739
4740               $foo = pack("a14","abcdefg");
4741               # "abcdefg\0\0\0\0\0\0\0"
4742
4743               $foo = pack("i9pl", gmtime);
4744               # a real struct tm (on my system anyway)
4745
4746               $utmp_template = "Z8 Z8 Z16 L";
4747               $utmp = pack($utmp_template, @utmp1);
4748               # a struct utmp (BSDish)
4749
4750               @utmp2 = unpack($utmp_template, $utmp);
4751               # "@utmp1" eq "@utmp2"
4752
4753               sub bintodec {
4754                   unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4755               }
4756
4757               $foo = pack('sx2l', 12, 34);
4758               # short 12, two zero bytes padding, long 34
4759               $bar = pack('s@4l', 12, 34);
4760               # short 12, zero fill to position 4, long 34
4761               # $foo eq $bar
4762               $baz = pack('s.l', 12, 4, 34);
4763               # short 12, zero fill to position 4, long 34
4764
4765               $foo = pack('nN', 42, 4711);
4766               # pack big-endian 16- and 32-bit unsigned integers
4767               $foo = pack('S>L>', 42, 4711);
4768               # exactly the same
4769               $foo = pack('s<l<', -42, 4711);
4770               # pack little-endian 16- and 32-bit signed integers
4771               $foo = pack('(sl)<', -42, 4711);
4772               # exactly the same
4773
4774           The same template may generally also be used in "unpack".
4775
4776       package NAMESPACE
4777       package NAMESPACE VERSION
4778       package NAMESPACE BLOCK
4779       package NAMESPACE VERSION BLOCK
4780           Declares the BLOCK or the rest of the compilation unit as being in
4781           the given namespace.  The scope of the package declaration is
4782           either the supplied code BLOCK or, in the absence of a BLOCK, from
4783           the declaration itself through the end of current scope (the
4784           enclosing block, file, or "eval").  That is, the forms without a
4785           BLOCK are operative through the end of the current scope, just like
4786           the "my", "state", and "our" operators.  All unqualified dynamic
4787           identifiers in this scope will be in the given namespace, except
4788           where overridden by another "package" declaration or when they're
4789           one of the special identifiers that qualify into "main::", like
4790           "STDOUT", "ARGV", "ENV", and the punctuation variables.
4791
4792           A package statement affects dynamic variables only, including those
4793           you've used "local" on, but not lexically-scoped variables, which
4794           are created with "my", "state", or "our".  Typically it would be
4795           the first declaration in a file included by "require" or "use".
4796           You can switch into a package in more than one place, since this
4797           only determines which default symbol table the compiler uses for
4798           the rest of that block.  You can refer to identifiers in other
4799           packages than the current one by prefixing the identifier with the
4800           package name and a double colon, as in $SomePack::var or
4801           "ThatPack::INPUT_HANDLE".  If package name is omitted, the "main"
4802           package is assumed.  That is, $::sail is equivalent to $main::sail
4803           (as well as to "$main'sail", still seen in ancient code, mostly
4804           from Perl 4).
4805
4806           If VERSION is provided, "package" sets the $VERSION variable in the
4807           given namespace to a version object with the VERSION provided.
4808           VERSION must be a "strict" style version number as defined by the
4809           version module: a positive decimal number (integer or decimal-
4810           fraction) without exponentiation or else a dotted-decimal v-string
4811           with a leading 'v' character and at least three components.  You
4812           should set $VERSION only once per package.
4813
4814           See "Packages" in perlmod for more information about packages,
4815           modules, and classes.  See perlsub for other scoping issues.
4816
4817       __PACKAGE__
4818           A special token that returns the name of the package in which it
4819           occurs.
4820
4821       pipe READHANDLE,WRITEHANDLE
4822           Opens a pair of connected pipes like the corresponding system call.
4823           Note that if you set up a loop of piped processes, deadlock can
4824           occur unless you are very careful.  In addition, note that Perl's
4825           pipes use IO buffering, so you may need to set $| to flush your
4826           WRITEHANDLE after each command, depending on the application.
4827
4828           Returns true on success.
4829
4830           See IPC::Open2, IPC::Open3, and "Bidirectional Communication with
4831           Another Process" in perlipc for examples of such things.
4832
4833           On systems that support a close-on-exec flag on files, that flag is
4834           set on all newly opened file descriptors whose "fileno"s are higher
4835           than the current value of $^F (by default 2 for "STDERR").  See
4836           "$^F" in perlvar.
4837
4838       pop ARRAY
4839       pop Pops and returns the last value of the array, shortening the array
4840           by one element.
4841
4842           Returns the undefined value if the array is empty, although this
4843           may also happen at other times.  If ARRAY is omitted, pops the
4844           @ARGV array in the main program, but the @_ array in subroutines,
4845           just like "shift".
4846
4847           Starting with Perl 5.14, an experimental feature allowed "pop" to
4848           take a scalar expression. This experiment has been deemed
4849           unsuccessful, and was removed as of Perl 5.24.
4850
4851       pos SCALAR
4852       pos Returns the offset of where the last "m//g" search left off for the
4853           variable in question ($_ is used when the variable is not
4854           specified).  This offset is in characters unless the (no-longer-
4855           recommended) "use bytes" pragma is in effect, in which case the
4856           offset is in bytes.  Note that 0 is a valid match offset.  "undef"
4857           indicates that the search position is reset (usually due to match
4858           failure, but can also be because no match has yet been run on the
4859           scalar).
4860
4861           "pos" directly accesses the location used by the regexp engine to
4862           store the offset, so assigning to "pos" will change that offset,
4863           and so will also influence the "\G" zero-width assertion in regular
4864           expressions.  Both of these effects take place for the next match,
4865           so you can't affect the position with "pos" during the current
4866           match, such as in "(?{pos() = 5})" or "s//pos() = 5/e".
4867
4868           Setting "pos" also resets the matched with zero-length flag,
4869           described under "Repeated Patterns Matching a Zero-length
4870           Substring" in perlre.
4871
4872           Because a failed "m//gc" match doesn't reset the offset, the return
4873           from "pos" won't change either in this case.  See perlre and
4874           perlop.
4875
4876       print FILEHANDLE LIST
4877       print FILEHANDLE
4878       print LIST
4879       print
4880           Prints a string or a list of strings.  Returns true if successful.
4881           FILEHANDLE may be a scalar variable containing the name of or a
4882           reference to the filehandle, thus introducing one level of
4883           indirection.  (NOTE: If FILEHANDLE is a variable and the next token
4884           is a term, it may be misinterpreted as an operator unless you
4885           interpose a "+" or put parentheses around the arguments.)  If
4886           FILEHANDLE is omitted, prints to the last selected (see "select")
4887           output handle.  If LIST is omitted, prints $_ to the currently
4888           selected output handle.  To use FILEHANDLE alone to print the
4889           content of $_ to it, you must use a bareword filehandle like "FH",
4890           not an indirect one like $fh.  To set the default output handle to
4891           something other than STDOUT, use the select operation.
4892
4893           The current value of $, (if any) is printed between each LIST item.
4894           The current value of "$\" (if any) is printed after the entire LIST
4895           has been printed.  Because print takes a LIST, anything in the LIST
4896           is evaluated in list context, including any subroutines whose
4897           return lists you pass to "print".  Be careful not to follow the
4898           print keyword with a left parenthesis unless you want the
4899           corresponding right parenthesis to terminate the arguments to the
4900           print; put parentheses around all arguments (or interpose a "+",
4901           but that doesn't look as good).
4902
4903           If you're storing handles in an array or hash, or in general
4904           whenever you're using any expression more complex than a bareword
4905           handle or a plain, unsubscripted scalar variable to retrieve it,
4906           you will have to use a block returning the filehandle value
4907           instead, in which case the LIST may not be omitted:
4908
4909               print { $files[$i] } "stuff\n";
4910               print { $OK ? *STDOUT : *STDERR } "stuff\n";
4911
4912           Printing to a closed pipe or socket will generate a SIGPIPE signal.
4913           See perlipc for more on signal handling.
4914
4915       printf FILEHANDLE FORMAT, LIST
4916       printf FILEHANDLE
4917       printf FORMAT, LIST
4918       printf
4919           Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except that
4920           "$\" (the output record separator) is not appended.  The FORMAT and
4921           the LIST are actually parsed as a single list.  The first argument
4922           of the list will be interpreted as the "printf" format.  This means
4923           that "printf(@_)" will use $_[0] as the format.  See sprintf for an
4924           explanation of the format argument.  If "use locale" (including
4925           "use locale ':not_characters'") is in effect and "POSIX::setlocale"
4926           has been called, the character used for the decimal separator in
4927           formatted floating-point numbers is affected by the "LC_NUMERIC"
4928           locale setting.  See perllocale and POSIX.
4929
4930           For historical reasons, if you omit the list, $_ is used as the
4931           format; to use FILEHANDLE without a list, you must use a bareword
4932           filehandle like "FH", not an indirect one like $fh.  However, this
4933           will rarely do what you want; if $_ contains formatting codes, they
4934           will be replaced with the empty string and a warning will be
4935           emitted if warnings are enabled.  Just use "print" if you want to
4936           print the contents of $_.
4937
4938           Don't fall into the trap of using a "printf" when a simple "print"
4939           would do.  The "print" is more efficient and less error prone.
4940
4941       prototype FUNCTION
4942       prototype
4943           Returns the prototype of a function as a string (or "undef" if the
4944           function has no prototype).  FUNCTION is a reference to, or the
4945           name of, the function whose prototype you want to retrieve.  If
4946           FUNCTION is omitted, $_ is used.
4947
4948           If FUNCTION is a string starting with "CORE::", the rest is taken
4949           as a name for a Perl builtin.  If the builtin's arguments cannot be
4950           adequately expressed by a prototype (such as "system"), "prototype"
4951           returns "undef", because the builtin does not really behave like a
4952           Perl function.  Otherwise, the string describing the equivalent
4953           prototype is returned.
4954
4955       push ARRAY,LIST
4956           Treats ARRAY as a stack by appending the values of LIST to the end
4957           of ARRAY.  The length of ARRAY increases by the length of LIST.
4958           Has the same effect as
4959
4960               for my $value (LIST) {
4961                   $ARRAY[++$#ARRAY] = $value;
4962               }
4963
4964           but is more efficient.  Returns the number of elements in the array
4965           following the completed "push".
4966
4967           Starting with Perl 5.14, an experimental feature allowed "push" to
4968           take a scalar expression. This experiment has been deemed
4969           unsuccessful, and was removed as of Perl 5.24.
4970
4971       q/STRING/
4972       qq/STRING/
4973       qw/STRING/
4974       qx/STRING/
4975           Generalized quotes.  See "Quote-Like Operators" in perlop.
4976
4977       qr/STRING/
4978           Regexp-like quote.  See "Regexp Quote-Like Operators" in perlop.
4979
4980       quotemeta EXPR
4981       quotemeta
4982           Returns the value of EXPR with all the ASCII non-"word" characters
4983           backslashed.  (That is, all ASCII characters not matching
4984           "/[A-Za-z_0-9]/" will be preceded by a backslash in the returned
4985           string, regardless of any locale settings.)  This is the internal
4986           function implementing the "\Q" escape in double-quoted strings.
4987           (See below for the behavior on non-ASCII code points.)
4988
4989           If EXPR is omitted, uses $_.
4990
4991           quotemeta (and "\Q" ... "\E") are useful when interpolating strings
4992           into regular expressions, because by default an interpolated
4993           variable will be considered a mini-regular expression.  For
4994           example:
4995
4996               my $sentence = 'The quick brown fox jumped over the lazy dog';
4997               my $substring = 'quick.*?fox';
4998               $sentence =~ s{$substring}{big bad wolf};
4999
5000           Will cause $sentence to become 'The big bad wolf jumped over...'.
5001
5002           On the other hand:
5003
5004               my $sentence = 'The quick brown fox jumped over the lazy dog';
5005               my $substring = 'quick.*?fox';
5006               $sentence =~ s{\Q$substring\E}{big bad wolf};
5007
5008           Or:
5009
5010               my $sentence = 'The quick brown fox jumped over the lazy dog';
5011               my $substring = 'quick.*?fox';
5012               my $quoted_substring = quotemeta($substring);
5013               $sentence =~ s{$quoted_substring}{big bad wolf};
5014
5015           Will both leave the sentence as is.  Normally, when accepting
5016           literal string input from the user, "quotemeta" or "\Q" must be
5017           used.
5018
5019           Beware that if you put literal backslashes (those not inside
5020           interpolated variables) between "\Q" and "\E", double-quotish
5021           backslash interpolation may lead to confusing results.  If you need
5022           to use literal backslashes within "\Q...\E", consult "Gory details
5023           of parsing quoted constructs" in perlop.
5024
5025           Because the result of "\Q STRING \E" has all metacharacters quoted,
5026           there is no way to insert a literal "$" or "@" inside a "\Q\E"
5027           pair.  If protected by "\", "$" will be quoted to become "\\\$"; if
5028           not, it is interpreted as the start of an interpolated scalar.
5029
5030           In Perl v5.14, all non-ASCII characters are quoted in
5031           non-UTF-8-encoded strings, but not quoted in UTF-8 strings.
5032
5033           Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5034           quoting non-ASCII characters; the quoting of ASCII characters is
5035           unchanged.
5036
5037           Also unchanged is the quoting of non-UTF-8 strings when outside the
5038           scope of a "use feature 'unicode_strings'", which is to quote all
5039           characters in the upper Latin1 range.  This provides complete
5040           backwards compatibility for old programs which do not use Unicode.
5041           (Note that "unicode_strings" is automatically enabled within the
5042           scope of a "use v5.12" or greater.)
5043
5044           Within the scope of "use locale", all non-ASCII Latin1 code points
5045           are quoted whether the string is encoded as UTF-8 or not.  As
5046           mentioned above, locale does not affect the quoting of ASCII-range
5047           characters.  This protects against those locales where characters
5048           such as "|" are considered to be word characters.
5049
5050           Otherwise, Perl quotes non-ASCII characters using an adaptation
5051           from Unicode (see <https://www.unicode.org/reports/tr31/>).  The
5052           only code points that are quoted are those that have any of the
5053           Unicode properties:  Pattern_Syntax, Pattern_White_Space,
5054           White_Space, Default_Ignorable_Code_Point, or
5055           General_Category=Control.
5056
5057           Of these properties, the two important ones are Pattern_Syntax and
5058           Pattern_White_Space.  They have been set up by Unicode for exactly
5059           this purpose of deciding which characters in a regular expression
5060           pattern should be quoted.  No character that can be in an
5061           identifier has these properties.
5062
5063           Perl promises, that if we ever add regular expression pattern
5064           metacharacters to the dozen already defined ("\ | ( ) [ { ^ $ * + ?
5065           ."), that we will only use ones that have the Pattern_Syntax
5066           property.  Perl also promises, that if we ever add characters that
5067           are considered to be white space in regular expressions (currently
5068           mostly affected by "/x"), they will all have the
5069           Pattern_White_Space property.
5070
5071           Unicode promises that the set of code points that have these two
5072           properties will never change, so something that is not quoted in
5073           v5.16 will never need to be quoted in any future Perl release.
5074           (Not all the code points that match Pattern_Syntax have actually
5075           had characters assigned to them; so there is room to grow, but they
5076           are quoted whether assigned or not.  Perl, of course, would never
5077           use an unassigned code point as an actual metacharacter.)
5078
5079           Quoting characters that have the other 3 properties is done to
5080           enhance the readability of the regular expression and not because
5081           they actually need to be quoted for regular expression purposes
5082           (characters with the White_Space property are likely to be
5083           indistinguishable on the page or screen from those with the
5084           Pattern_White_Space property; and the other two properties contain
5085           non-printing characters).
5086
5087       rand EXPR
5088       rand
5089           Returns a random fractional number greater than or equal to 0 and
5090           less than the value of EXPR.  (EXPR should be positive.)  If EXPR
5091           is omitted, the value 1 is used.  Currently EXPR with the value 0
5092           is also special-cased as 1 (this was undocumented before Perl 5.8.0
5093           and is subject to change in future versions of Perl).
5094           Automatically calls "srand" unless "srand" has already been called.
5095           See also "srand".
5096
5097           Apply "int" to the value returned by "rand" if you want random
5098           integers instead of random fractional numbers.  For example,
5099
5100               int(rand(10))
5101
5102           returns a random integer between 0 and 9, inclusive.
5103
5104           (Note: If your rand function consistently returns numbers that are
5105           too large or too small, then your version of Perl was probably
5106           compiled with the wrong number of RANDBITS.)
5107
5108           "rand" is not cryptographically secure.  You should not rely on it
5109           in security-sensitive situations.  As of this writing, a number of
5110           third-party CPAN modules offer random number generators intended by
5111           their authors to be cryptographically secure, including:
5112           Data::Entropy, Crypt::Random, Math::Random::Secure, and
5113           Math::TrulyRandom.
5114
5115       read FILEHANDLE,SCALAR,LENGTH,OFFSET
5116       read FILEHANDLE,SCALAR,LENGTH
5117           Attempts to read LENGTH characters of data into variable SCALAR
5118           from the specified FILEHANDLE.  Returns the number of characters
5119           actually read, 0 at end of file, or undef if there was an error (in
5120           the latter case $! is also set).  SCALAR will be grown or shrunk so
5121           that the last character actually read is the last character of the
5122           scalar after the read.
5123
5124           An OFFSET may be specified to place the read data at some place in
5125           the string other than the beginning.  A negative OFFSET specifies
5126           placement at that many characters counting backwards from the end
5127           of the string.  A positive OFFSET greater than the length of SCALAR
5128           results in the string being padded to the required size with "\0"
5129           bytes before the result of the read is appended.
5130
5131           The call is implemented in terms of either Perl's or your system's
5132           native fread(3) library function, via the PerlIO layers applied to
5133           the handle.  To get a true read(2) system call, see sysread.
5134
5135           Note the characters: depending on the status of the filehandle,
5136           either (8-bit) bytes or characters are read.  By default, all
5137           filehandles operate on bytes, but for example if the filehandle has
5138           been opened with the ":utf8" I/O layer (see "open", and the open
5139           pragma), the I/O will operate on UTF8-encoded Unicode characters,
5140           not bytes.  Similarly for the ":encoding" layer: in that case
5141           pretty much any characters can be read.
5142
5143       readdir DIRHANDLE
5144           Returns the next directory entry for a directory opened by
5145           "opendir".  If used in list context, returns all the rest of the
5146           entries in the directory.  If there are no more entries, returns
5147           the undefined value in scalar context and the empty list in list
5148           context.
5149
5150           If you're planning to filetest the return values out of a
5151           "readdir", you'd better prepend the directory in question.
5152           Otherwise, because we didn't "chdir" there, it would have been
5153           testing the wrong file.
5154
5155               opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
5156               my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5157               closedir $dh;
5158
5159           As of Perl 5.12 you can use a bare "readdir" in a "while" loop,
5160           which will set $_ on every iteration.  If either a "readdir"
5161           expression or an explicit assignment of a "readdir" expression to a
5162           scalar is used as a "while"/"for" condition, then the condition
5163           actually tests for definedness of the expression's value, not for
5164           its regular truth value.
5165
5166               opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
5167               while (readdir $dh) {
5168                   print "$some_dir/$_\n";
5169               }
5170               closedir $dh;
5171
5172           To avoid confusing would-be users of your code who are running
5173           earlier versions of Perl with mysterious failures, put this sort of
5174           thing at the top of your file to signal that your code will work
5175           only on Perls of a recent vintage:
5176
5177               use 5.012; # so readdir assigns to $_ in a lone while test
5178
5179       readline EXPR
5180       readline
5181           Reads from the filehandle whose typeglob is contained in EXPR (or
5182           from *ARGV if EXPR is not provided).  In scalar context, each call
5183           reads and returns the next line until end-of-file is reached,
5184           whereupon the subsequent call returns "undef".  In list context,
5185           reads until end-of-file is reached and returns a list of lines.
5186           Note that the notion of "line" used here is whatever you may have
5187           defined with $/ (or $INPUT_RECORD_SEPARATOR in English).  See "$/"
5188           in perlvar.
5189
5190           When $/ is set to "undef", when "readline" is in scalar context
5191           (i.e., file slurp mode), and when an empty file is read, it returns
5192           '' the first time, followed by "undef" subsequently.
5193
5194           This is the internal function implementing the "<EXPR>" operator,
5195           but you can use it directly.  The "<EXPR>" operator is discussed in
5196           more detail in "I/O Operators" in perlop.
5197
5198               my $line = <STDIN>;
5199               my $line = readline(STDIN);    # same thing
5200
5201           If "readline" encounters an operating system error, $! will be set
5202           with the corresponding error message.  It can be helpful to check
5203           $! when you are reading from filehandles you don't trust, such as a
5204           tty or a socket.  The following example uses the operator form of
5205           "readline" and dies if the result is not defined.
5206
5207               while ( ! eof($fh) ) {
5208                   defined( $_ = readline $fh ) or die "readline failed: $!";
5209                   ...
5210               }
5211
5212           Note that you have can't handle "readline" errors that way with the
5213           "ARGV" filehandle.  In that case, you have to open each element of
5214           @ARGV yourself since "eof" handles "ARGV" differently.
5215
5216               foreach my $arg (@ARGV) {
5217                   open(my $fh, $arg) or warn "Can't open $arg: $!";
5218
5219                   while ( ! eof($fh) ) {
5220                       defined( $_ = readline $fh )
5221                           or die "readline failed for $arg: $!";
5222                       ...
5223                   }
5224               }
5225
5226           Like the "<EXPR>" operator, if a "readline" expression is used as
5227           the condition of a "while" or "for" loop, then it will be
5228           implicitly assigned to $_.  If either a "readline" expression or an
5229           explicit assignment of a "readline" expression to a scalar is used
5230           as a "while"/"for" condition, then the condition actually tests for
5231           definedness of the expression's value, not for its regular truth
5232           value.
5233
5234       readlink EXPR
5235       readlink
5236           Returns the value of a symbolic link, if symbolic links are
5237           implemented.  If not, raises an exception.  If there is a system
5238           error, returns the undefined value and sets $! (errno).  If EXPR is
5239           omitted, uses $_.
5240
5241           Portability issues: "readlink" in perlport.
5242
5243       readpipe EXPR
5244       readpipe
5245           EXPR is executed as a system command.  The collected standard
5246           output of the command is returned.  In scalar context, it comes
5247           back as a single (potentially multi-line) string.  In list context,
5248           returns a list of lines (however you've defined lines with $/ (or
5249           $INPUT_RECORD_SEPARATOR in English)).  This is the internal
5250           function implementing the "qx/EXPR/" operator, but you can use it
5251           directly.  The "qx/EXPR/" operator is discussed in more detail in
5252           ""qx/STRING/"" in perlop.  If EXPR is omitted, uses $_.
5253
5254       recv SOCKET,SCALAR,LENGTH,FLAGS
5255           Receives a message on a socket.  Attempts to receive LENGTH
5256           characters of data into variable SCALAR from the specified SOCKET
5257           filehandle.  SCALAR will be grown or shrunk to the length actually
5258           read.  Takes the same flags as the system call of the same name.
5259           Returns the address of the sender if SOCKET's protocol supports
5260           this; returns an empty string otherwise.  If there's an error,
5261           returns the undefined value.  This call is actually implemented in
5262           terms of the recvfrom(2) system call.  See "UDP: Message Passing"
5263           in perlipc for examples.
5264
5265           Note that if the socket has been marked as ":utf8", "recv" will
5266           throw an exception.  The ":encoding(...)" layer implicitly
5267           introduces the ":utf8" layer.  See "binmode".
5268
5269       redo LABEL
5270       redo EXPR
5271       redo
5272           The "redo" command restarts the loop block without evaluating the
5273           conditional again.  The "continue" block, if any, is not executed.
5274           If the LABEL is omitted, the command refers to the innermost
5275           enclosing loop.  The "redo EXPR" form, available starting in Perl
5276           5.18.0, allows a label name to be computed at run time, and is
5277           otherwise identical to "redo LABEL".  Programs that want to lie to
5278           themselves about what was just input normally use this command:
5279
5280               # a simpleminded Pascal comment stripper
5281               # (warning: assumes no { or } in strings)
5282               LINE: while (<STDIN>) {
5283                   while (s|({.*}.*){.*}|$1 |) {}
5284                   s|{.*}| |;
5285                   if (s|{.*| |) {
5286                       my $front = $_;
5287                       while (<STDIN>) {
5288                           if (/}/) {  # end of comment?
5289                               s|^|$front\{|;
5290                               redo LINE;
5291                           }
5292                       }
5293                   }
5294                   print;
5295               }
5296
5297           "redo" cannot return a value from a block that typically returns a
5298           value, such as "eval {}", "sub {}", or "do {}". It will perform its
5299           flow control behavior, which precludes any return value. It should
5300           not be used to exit a "grep" or "map" operation.
5301
5302           Note that a block by itself is semantically identical to a loop
5303           that executes once.  Thus "redo" inside such a block will
5304           effectively turn it into a looping construct.
5305
5306           See also "continue" for an illustration of how "last", "next", and
5307           "redo" work.
5308
5309           Unlike most named operators, this has the same precedence as
5310           assignment.  It is also exempt from the looks-like-a-function rule,
5311           so "redo ("foo")."bar"" will cause "bar" to be part of the argument
5312           to "redo".
5313
5314       ref EXPR
5315       ref Examines the value of EXPR, expecting it to be a reference, and
5316           returns a string giving information about the reference and the
5317           type of referent.  If EXPR is not specified, $_ will be used.
5318
5319           If the operand is not a reference, then the empty string will be
5320           returned.  An empty string will only be returned in this situation.
5321           "ref" is often useful to just test whether a value is a reference,
5322           which can be done by comparing the result to the empty string.  It
5323           is a common mistake to use the result of "ref" directly as a truth
5324           value: this goes wrong because 0 (which is false) can be returned
5325           for a reference.
5326
5327           If the operand is a reference to a blessed object, then the name of
5328           the class into which the referent is blessed will be returned.
5329           "ref" doesn't care what the physical type of the referent is;
5330           blessing takes precedence over such concerns.  Beware that exact
5331           comparison of "ref" results against a class name doesn't perform a
5332           class membership test: a class's members also include objects
5333           blessed into subclasses, for which "ref" will return the name of
5334           the subclass.  Also beware that class names can clash with the
5335           built-in type names (described below).
5336
5337           If the operand is a reference to an unblessed object, then the
5338           return value indicates the type of object.  If the unblessed
5339           referent is not a scalar, then the return value will be one of the
5340           strings "ARRAY", "HASH", "CODE", "FORMAT", or "IO", indicating only
5341           which kind of object it is.  If the unblessed referent is a scalar,
5342           then the return value will be one of the strings "SCALAR",
5343           "VSTRING", "REF", "GLOB", "LVALUE", or "REGEXP", depending on the
5344           kind of value the scalar currently has.   But note that "qr//"
5345           scalars are created already blessed, so "ref qr/.../" will likely
5346           return "Regexp".  Beware that these built-in type names can also be
5347           used as class names, so "ref" returning one of these names doesn't
5348           unambiguously indicate that the referent is of the kind to which
5349           the name refers.
5350
5351           The ambiguity between built-in type names and class names
5352           significantly limits the utility of "ref".  For unambiguous
5353           information, use "Scalar::Util::blessed()" for information about
5354           blessing, and "Scalar::Util::reftype()" for information about
5355           physical types.  Use the "isa" method for class membership tests,
5356           though one must be sure of blessedness before attempting a method
5357           call.
5358
5359           See also perlref and perlobj.
5360
5361       rename OLDNAME,NEWNAME
5362           Changes the name of a file; an existing file NEWNAME will be
5363           clobbered.  Returns true for success; on failure returns false and
5364           sets $!.
5365
5366           Behavior of this function varies wildly depending on your system
5367           implementation.  For example, it will usually not work across file
5368           system boundaries, even though the system mv command sometimes
5369           compensates for this.  Other restrictions include whether it works
5370           on directories, open files, or pre-existing files.  Check perlport
5371           and either the rename(2) manpage or equivalent system documentation
5372           for details.
5373
5374           For a platform independent "move" function look at the File::Copy
5375           module.
5376
5377           Portability issues: "rename" in perlport.
5378
5379       require VERSION
5380       require EXPR
5381       require
5382           Demands a version of Perl specified by VERSION, or demands some
5383           semantics specified by EXPR or by $_ if EXPR is not supplied.
5384
5385           VERSION may be either a literal such as v5.24.1, which will be
5386           compared to $^V (or $PERL_VERSION in English), or a numeric
5387           argument of the form 5.024001, which will be compared to $]. An
5388           exception is raised if VERSION is greater than the version of the
5389           current Perl interpreter.  Compare with "use", which can do a
5390           similar check at compile time.
5391
5392           Specifying VERSION as a numeric argument of the form 5.024001
5393           should generally be avoided as older less readable syntax compared
5394           to v5.24.1. Before perl 5.8.0 (released in 2002), the more verbose
5395           numeric form was the only supported syntax, which is why you might
5396           see it in older code.
5397
5398               require v5.24.1;    # run time version check
5399               require 5.24.1;     # ditto
5400               require 5.024_001;  # ditto; older syntax compatible
5401                                     with perl 5.6
5402
5403           Otherwise, "require" demands that a library file be included if it
5404           hasn't already been included.  The file is included via the do-FILE
5405           mechanism, which is essentially just a variety of "eval" with the
5406           caveat that lexical variables in the invoking script will be
5407           invisible to the included code.  If it were implemented in pure
5408           Perl, it would have semantics similar to the following:
5409
5410               use Carp 'croak';
5411               use version;
5412
5413               sub require {
5414                   my ($filename) = @_;
5415                   if ( my $version = eval { version->parse($filename) } ) {
5416                       if ( $version > $^V ) {
5417                          my $vn = $version->normal;
5418                          croak "Perl $vn required--this is only $^V, stopped";
5419                       }
5420                       return 1;
5421                   }
5422
5423                   if (exists $INC{$filename}) {
5424                       return 1 if $INC{$filename};
5425                       croak "Compilation failed in require";
5426                   }
5427
5428                   foreach $prefix (@INC) {
5429                       if (ref($prefix)) {
5430                           #... do other stuff - see text below ....
5431                       }
5432                       # (see text below about possible appending of .pmc
5433                       # suffix to $filename)
5434                       my $realfilename = "$prefix/$filename";
5435                       next if ! -e $realfilename || -d _ || -b _;
5436                       $INC{$filename} = $realfilename;
5437                       my $result = do($realfilename);
5438                                    # but run in caller's namespace
5439
5440                       if (!defined $result) {
5441                           $INC{$filename} = undef;
5442                           croak $@ ? "$@Compilation failed in require"
5443                                    : "Can't locate $filename: $!\n";
5444                       }
5445                       if (!$result) {
5446                           delete $INC{$filename};
5447                           croak "$filename did not return true value";
5448                       }
5449                       $! = 0;
5450                       return $result;
5451                   }
5452                   croak "Can't locate $filename in \@INC ...";
5453               }
5454
5455           Note that the file will not be included twice under the same
5456           specified name.
5457
5458           The file must return true as the last statement to indicate
5459           successful execution of any initialization code, so it's customary
5460           to end such a file with "1;" unless you're sure it'll return true
5461           otherwise.  But it's better just to put the "1;", in case you add
5462           more statements.
5463
5464           If EXPR is a bareword, "require" assumes a .pm extension and
5465           replaces "::" with "/" in the filename for you, to make it easy to
5466           load standard modules.  This form of loading of modules does not
5467           risk altering your namespace, however it will autovivify the stash
5468           for the required module.
5469
5470           In other words, if you try this:
5471
5472                   require Foo::Bar;     # a splendid bareword
5473
5474           The require function will actually look for the Foo/Bar.pm file in
5475           the directories specified in the @INC array, and it will autovivify
5476           the "Foo::Bar::" stash at compile time.
5477
5478           But if you try this:
5479
5480                   my $class = 'Foo::Bar';
5481                   require $class;       # $class is not a bareword
5482               #or
5483                   require "Foo::Bar";   # not a bareword because of the ""
5484
5485           The require function will look for the Foo::Bar file in the @INC
5486           array and will complain about not finding Foo::Bar there.  In this
5487           case you can do:
5488
5489                   eval "require $class";
5490
5491           or you could do
5492
5493                   require "Foo/Bar.pm";
5494
5495           Neither of these forms will autovivify any stashes at compile time
5496           and only have run time effects.
5497
5498           Now that you understand how "require" looks for files with a
5499           bareword argument, there is a little extra functionality going on
5500           behind the scenes.  Before "require" looks for a .pm extension, it
5501           will first look for a similar filename with a .pmc extension.  If
5502           this file is found, it will be loaded in place of any file ending
5503           in a .pm extension. This applies to both the explicit "require
5504           "Foo/Bar.pm";" form and the "require Foo::Bar;" form.
5505
5506           You can also insert hooks into the import facility by putting Perl
5507           code directly into the @INC array.  There are three forms of hooks:
5508           subroutine references, array references, and blessed objects.
5509
5510           Subroutine references are the simplest case.  When the inclusion
5511           system walks through @INC and encounters a subroutine, this
5512           subroutine gets called with two parameters, the first a reference
5513           to itself, and the second the name of the file to be included
5514           (e.g., Foo/Bar.pm).  The subroutine should return either nothing or
5515           else a list of up to four values in the following order:
5516
5517           1.  A reference to a scalar, containing any initial source code to
5518               prepend to the file or generator output.
5519
5520           2.  A filehandle, from which the file will be read.
5521
5522           3.  A reference to a subroutine.  If there is no filehandle
5523               (previous item), then this subroutine is expected to generate
5524               one line of source code per call, writing the line into $_ and
5525               returning 1, then finally at end of file returning 0.  If there
5526               is a filehandle, then the subroutine will be called to act as a
5527               simple source filter, with the line as read in $_.  Again,
5528               return 1 for each valid line, and 0 after all lines have been
5529               returned.  For historical reasons the subroutine will receive a
5530               meaningless argument (in fact always the numeric value zero) as
5531               $_[0].
5532
5533           4.  Optional state for the subroutine.  The state is passed in as
5534               $_[1].
5535
5536           If an empty list, "undef", or nothing that matches the first 3
5537           values above is returned, then "require" looks at the remaining
5538           elements of @INC.  Note that this filehandle must be a real
5539           filehandle (strictly a typeglob or reference to a typeglob, whether
5540           blessed or unblessed); tied filehandles will be ignored and
5541           processing will stop there.
5542
5543           If the hook is an array reference, its first element must be a
5544           subroutine reference.  This subroutine is called as above, but the
5545           first parameter is the array reference.  This lets you indirectly
5546           pass arguments to the subroutine.
5547
5548           In other words, you can write:
5549
5550               push @INC, \&my_sub;
5551               sub my_sub {
5552                   my ($coderef, $filename) = @_;  # $coderef is \&my_sub
5553                   ...
5554               }
5555
5556           or:
5557
5558               push @INC, [ \&my_sub, $x, $y, ... ];
5559               sub my_sub {
5560                   my ($arrayref, $filename) = @_;
5561                   # Retrieve $x, $y, ...
5562                   my (undef, @parameters) = @$arrayref;
5563                   ...
5564               }
5565
5566           If the hook is an object, it must provide an "INC" method that will
5567           be called as above, the first parameter being the object itself.
5568           (Note that you must fully qualify the sub's name, as unqualified
5569           "INC" is always forced into package "main".)  Here is a typical
5570           code layout:
5571
5572               # In Foo.pm
5573               package Foo;
5574               sub new { ... }
5575               sub Foo::INC {
5576                   my ($self, $filename) = @_;
5577                   ...
5578               }
5579
5580               # In the main program
5581               push @INC, Foo->new(...);
5582
5583           These hooks are also permitted to set the %INC entry corresponding
5584           to the files they have loaded.  See "%INC" in perlvar.
5585
5586           For a yet-more-powerful import facility, see "use" and perlmod.
5587
5588       reset EXPR
5589       reset
5590           Generally used in a "continue" block at the end of a loop to clear
5591           variables and reset "m?pattern?" searches so that they work again.
5592           The expression is interpreted as a list of single characters
5593           (hyphens allowed for ranges).  All variables (scalars, arrays, and
5594           hashes) in the current package beginning with one of those letters
5595           are reset to their pristine state.  If the expression is omitted,
5596           one-match searches ("m?pattern?") are reset to match again.  Only
5597           resets variables or searches in the current package.  Always
5598           returns 1.  Examples:
5599
5600               reset 'X';      # reset all X variables
5601               reset 'a-z';    # reset lower case variables
5602               reset;          # just reset m?one-time? searches
5603
5604           Resetting "A-Z" is not recommended because you'll wipe out your
5605           @ARGV and @INC arrays and your %ENV hash.
5606
5607           Resets only package variables; lexical variables are unaffected,
5608           but they clean themselves up on scope exit anyway, so you'll
5609           probably want to use them instead.  See "my".
5610
5611       return EXPR
5612       return
5613           Returns from a subroutine, "eval", "do FILE", "sort" block or regex
5614           eval block (but not a "grep", "map", or "do BLOCK" block) with the
5615           value given in EXPR.  Evaluation of EXPR may be in list, scalar, or
5616           void context, depending on how the return value will be used, and
5617           the context may vary from one execution to the next (see
5618           "wantarray").  If no EXPR is given, returns an empty list in list
5619           context, the undefined value in scalar context, and (of course)
5620           nothing at all in void context.
5621
5622           (In the absence of an explicit "return", a subroutine, "eval", or
5623           "do FILE" automatically returns the value of the last expression
5624           evaluated.)
5625
5626           Unlike most named operators, this is also exempt from the looks-
5627           like-a-function rule, so "return ("foo")."bar"" will cause "bar" to
5628           be part of the argument to "return".
5629
5630       reverse LIST
5631           In list context, returns a list value consisting of the elements of
5632           LIST in the opposite order.  In scalar context, concatenates the
5633           elements of LIST and returns a string value with all characters in
5634           the opposite order.
5635
5636               print join(", ", reverse "world", "Hello"); # Hello, world
5637
5638               print scalar reverse "dlrow ,", "olleH";    # Hello, world
5639
5640           Used without arguments in scalar context, "reverse" reverses $_.
5641
5642               $_ = "dlrow ,olleH";
5643               print reverse;                         # No output, list context
5644               print scalar reverse;                  # Hello, world
5645
5646           Note that reversing an array to itself (as in "@a = reverse @a")
5647           will preserve non-existent elements whenever possible; i.e., for
5648           non-magical arrays or for tied arrays with "EXISTS" and "DELETE"
5649           methods.
5650
5651           This operator is also handy for inverting a hash, although there
5652           are some caveats.  If a value is duplicated in the original hash,
5653           only one of those can be represented as a key in the inverted hash.
5654           Also, this has to unwind one hash and build a whole new one, which
5655           may take some time on a large hash, such as from a DBM file.
5656
5657               my %by_name = reverse %by_address;  # Invert the hash
5658
5659       rewinddir DIRHANDLE
5660           Sets the current position to the beginning of the directory for the
5661           "readdir" routine on DIRHANDLE.
5662
5663           Portability issues: "rewinddir" in perlport.
5664
5665       rindex STR,SUBSTR,POSITION
5666       rindex STR,SUBSTR
5667           Works just like "index" except that it returns the position of the
5668           last occurrence of SUBSTR in STR.  If POSITION is specified,
5669           returns the last occurrence beginning at or before that position.
5670
5671       rmdir FILENAME
5672       rmdir
5673           Deletes the directory specified by FILENAME if that directory is
5674           empty.  If it succeeds it returns true; otherwise it returns false
5675           and sets $! (errno).  If FILENAME is omitted, uses $_.
5676
5677           To remove a directory tree recursively ("rm -rf" on Unix) look at
5678           the "rmtree" function of the File::Path module.
5679
5680       s///
5681           The substitution operator.  See "Regexp Quote-Like Operators" in
5682           perlop.
5683
5684       say FILEHANDLE LIST
5685       say FILEHANDLE
5686       say LIST
5687       say Just like "print", but implicitly appends a newline at the end of
5688           the LIST instead of any value "$\" might have.  To use FILEHANDLE
5689           without a LIST to print the contents of $_ to it, you must use a
5690           bareword filehandle like "FH", not an indirect one like $fh.
5691
5692           "say" is available only if the "say" feature is enabled or if it is
5693           prefixed with "CORE::".  The "say" feature is enabled automatically
5694           with a "use v5.10" (or higher) declaration in the current scope.
5695
5696       scalar EXPR
5697           Forces EXPR to be interpreted in scalar context and returns the
5698           value of EXPR.
5699
5700               my @counts = ( scalar @a, scalar @b, scalar @c );
5701
5702           There is no equivalent operator to force an expression to be
5703           interpolated in list context because in practice, this is never
5704           needed.  If you really wanted to do so, however, you could use the
5705           construction "@{[ (some expression) ]}", but usually a simple
5706           "(some expression)" suffices.
5707
5708           Because "scalar" is a unary operator, if you accidentally use a
5709           parenthesized list for the EXPR, this behaves as a scalar comma
5710           expression, evaluating all but the last element in void context and
5711           returning the final element evaluated in scalar context.  This is
5712           seldom what you want.
5713
5714           The following single statement:
5715
5716               print uc(scalar(foo(), $bar)), $baz;
5717
5718           is the moral equivalent of these two:
5719
5720               foo();
5721               print(uc($bar), $baz);
5722
5723           See perlop for more details on unary operators and the comma
5724           operator, and perldata for details on evaluating a hash in scalar
5725           context.
5726
5727       seek FILEHANDLE,POSITION,WHENCE
5728           Sets FILEHANDLE's position, just like the fseek(3) call of C
5729           "stdio".  FILEHANDLE may be an expression whose value gives the
5730           name of the filehandle.  The values for WHENCE are 0 to set the new
5731           position in bytes to POSITION; 1 to set it to the current position
5732           plus POSITION; and 2 to set it to EOF plus POSITION, typically
5733           negative.  For WHENCE you may use the constants "SEEK_SET",
5734           "SEEK_CUR", and "SEEK_END" (start of the file, current position,
5735           end of the file) from the Fcntl module.  Returns 1 on success,
5736           false otherwise.
5737
5738           Note the emphasis on bytes: even if the filehandle has been set to
5739           operate on characters (for example using the ":encoding(UTF-8)" I/O
5740           layer), the "seek", "tell", and "sysseek" family of functions use
5741           byte offsets, not character offsets, because seeking to a character
5742           offset would be very slow in a UTF-8 file.
5743
5744           If you want to position the file for "sysread" or "syswrite", don't
5745           use "seek", because buffering makes its effect on the file's read-
5746           write position unpredictable and non-portable.  Use "sysseek"
5747           instead.
5748
5749           Due to the rules and rigors of ANSI C, on some systems you have to
5750           do a seek whenever you switch between reading and writing.  Amongst
5751           other things, this may have the effect of calling stdio's
5752           clearerr(3).  A WHENCE of 1 ("SEEK_CUR") is useful for not moving
5753           the file position:
5754
5755               seek($fh, 0, 1);
5756
5757           This is also useful for applications emulating "tail -f".  Once you
5758           hit EOF on your read and then sleep for a while, you (probably)
5759           have to stick in a dummy "seek" to reset things.  The "seek"
5760           doesn't change the position, but it does clear the end-of-file
5761           condition on the handle, so that the next "readline FILE" makes
5762           Perl try again to read something.  (We hope.)
5763
5764           If that doesn't work (some I/O implementations are particularly
5765           cantankerous), you might need something like this:
5766
5767               for (;;) {
5768                   for ($curpos = tell($fh); $_ = readline($fh);
5769                        $curpos = tell($fh)) {
5770                       # search for some stuff and put it into files
5771                   }
5772                   sleep($for_a_while);
5773                   seek($fh, $curpos, 0);
5774               }
5775
5776       seekdir DIRHANDLE,POS
5777           Sets the current position for the "readdir" routine on DIRHANDLE.
5778           POS must be a value returned by "telldir".  "seekdir" also has the
5779           same caveats about possible directory compaction as the
5780           corresponding system library routine.
5781
5782       select FILEHANDLE
5783       select
5784           Returns the currently selected filehandle.  If FILEHANDLE is
5785           supplied, sets the new current default filehandle for output.  This
5786           has two effects: first, a "write" or a "print" without a filehandle
5787           default to this FILEHANDLE.  Second, references to variables
5788           related to output will refer to this output channel.
5789
5790           For example, to set the top-of-form format for more than one output
5791           channel, you might do the following:
5792
5793               select(REPORT1);
5794               $^ = 'report1_top';
5795               select(REPORT2);
5796               $^ = 'report2_top';
5797
5798           FILEHANDLE may be an expression whose value gives the name of the
5799           actual filehandle.  Thus:
5800
5801               my $oldfh = select(STDERR); $| = 1; select($oldfh);
5802
5803           Some programmers may prefer to think of filehandles as objects with
5804           methods, preferring to write the last example as:
5805
5806               STDERR->autoflush(1);
5807
5808           (Prior to Perl version 5.14, you have to "use IO::Handle;"
5809           explicitly first.)
5810
5811           Portability issues: "select" in perlport.
5812
5813       select RBITS,WBITS,EBITS,TIMEOUT
5814           This calls the select(2) syscall with the bit masks specified,
5815           which can be constructed using "fileno" and "vec", along these
5816           lines:
5817
5818               my $rin = my $win = my $ein = '';
5819               vec($rin, fileno(STDIN),  1) = 1;
5820               vec($win, fileno(STDOUT), 1) = 1;
5821               $ein = $rin | $win;
5822
5823           If you want to select on many filehandles, you may wish to write a
5824           subroutine like this:
5825
5826               sub fhbits {
5827                   my @fhlist = @_;
5828                   my $bits = "";
5829                   for my $fh (@fhlist) {
5830                       vec($bits, fileno($fh), 1) = 1;
5831                   }
5832                   return $bits;
5833               }
5834               my $rin = fhbits(\*STDIN, $tty, $mysock);
5835
5836           The usual idiom is:
5837
5838            my ($nfound, $timeleft) =
5839              select(my $rout = $rin, my $wout = $win, my $eout = $ein,
5840                                                                     $timeout);
5841
5842           or to block until something becomes ready just do this
5843
5844            my $nfound =
5845              select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
5846
5847           Most systems do not bother to return anything useful in $timeleft,
5848           so calling "select" in scalar context just returns $nfound.
5849
5850           Any of the bit masks can also be "undef".  The timeout, if
5851           specified, is in seconds, which may be fractional.  Note: not all
5852           implementations are capable of returning the $timeleft.  If not,
5853           they always return $timeleft equal to the supplied $timeout.
5854
5855           You can effect a sleep of 250 milliseconds this way:
5856
5857               select(undef, undef, undef, 0.25);
5858
5859           Note that whether "select" gets restarted after signals (say,
5860           SIGALRM) is implementation-dependent.  See also perlport for notes
5861           on the portability of "select".
5862
5863           On error, "select" behaves just like select(2): it returns "-1" and
5864           sets $!.
5865
5866           On some Unixes, select(2) may report a socket file descriptor as
5867           "ready for reading" even when no data is available, and thus any
5868           subsequent "read" would block.  This can be avoided if you always
5869           use "O_NONBLOCK" on the socket.  See select(2) and fcntl(2) for
5870           further details.
5871
5872           The standard "IO::Select" module provides a user-friendlier
5873           interface to "select", mostly because it does all the bit-mask work
5874           for you.
5875
5876           WARNING: One should not attempt to mix buffered I/O (like "read" or
5877           "readline") with "select", except as permitted by POSIX, and even
5878           then only on POSIX systems.  You have to use "sysread" instead.
5879
5880           Portability issues: "select" in perlport.
5881
5882       semctl ID,SEMNUM,CMD,ARG
5883           Calls the System V IPC function semctl(2).  You'll probably have to
5884           say
5885
5886               use IPC::SysV;
5887
5888           first to get the correct constant definitions.  If CMD is IPC_STAT
5889           or GETALL, then ARG must be a variable that will hold the returned
5890           semid_ds structure or semaphore value array.  Returns like "ioctl":
5891           the undefined value for error, ""0 but true"" for zero, or the
5892           actual return value otherwise.  The ARG must consist of a vector of
5893           native short integers, which may be created with
5894           "pack("s!",(0)x$nsem)".  See also "SysV IPC" in perlipc and the
5895           documentation for "IPC::SysV" and "IPC::Semaphore".
5896
5897           Portability issues: "semctl" in perlport.
5898
5899       semget KEY,NSEMS,FLAGS
5900           Calls the System V IPC function semget(2).  Returns the semaphore
5901           id, or the undefined value on error.  See also "SysV IPC" in
5902           perlipc and the documentation for "IPC::SysV" and "IPC::Semaphore".
5903
5904           Portability issues: "semget" in perlport.
5905
5906       semop KEY,OPSTRING
5907           Calls the System V IPC function semop(2) for semaphore operations
5908           such as signalling and waiting.  OPSTRING must be a packed array of
5909           semop structures.  Each semop structure can be generated with
5910           "pack("s!3", $semnum, $semop, $semflag)".  The length of OPSTRING
5911           implies the number of semaphore operations.  Returns true if
5912           successful, false on error.  As an example, the following code
5913           waits on semaphore $semnum of semaphore id $semid:
5914
5915               my $semop = pack("s!3", $semnum, -1, 0);
5916               die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5917
5918           To signal the semaphore, replace "-1" with 1.  See also "SysV IPC"
5919           in perlipc and the documentation for "IPC::SysV" and
5920           "IPC::Semaphore".
5921
5922           Portability issues: "semop" in perlport.
5923
5924       send SOCKET,MSG,FLAGS,TO
5925       send SOCKET,MSG,FLAGS
5926           Sends a message on a socket.  Attempts to send the scalar MSG to
5927           the SOCKET filehandle.  Takes the same flags as the system call of
5928           the same name.  On unconnected sockets, you must specify a
5929           destination to send to, in which case it does a sendto(2) syscall.
5930           Returns the number of characters sent, or the undefined value on
5931           error.  The sendmsg(2) syscall is currently unimplemented.  See
5932           "UDP: Message Passing" in perlipc for examples.
5933
5934           Note that if the socket has been marked as ":utf8", "send" will
5935           throw an exception.  The ":encoding(...)" layer implicitly
5936           introduces the ":utf8" layer.  See "binmode".
5937
5938       setpgrp PID,PGRP
5939           Sets the current process group for the specified PID, 0 for the
5940           current process.  Raises an exception when used on a machine that
5941           doesn't implement POSIX setpgid(2) or BSD setpgrp(2).  If the
5942           arguments are omitted, it defaults to "0,0".  Note that the BSD 4.2
5943           version of "setpgrp" does not accept any arguments, so only
5944           "setpgrp(0,0)" is portable.  See also "POSIX::setsid()".
5945
5946           Portability issues: "setpgrp" in perlport.
5947
5948       setpriority WHICH,WHO,PRIORITY
5949           Sets the current priority for a process, a process group, or a
5950           user.  (See setpriority(2).)  Raises an exception when used on a
5951           machine that doesn't implement setpriority(2).
5952
5953           "WHICH" can be any of "PRIO_PROCESS", "PRIO_PGRP" or "PRIO_USER"
5954           imported from "RESOURCE CONSTANTS" in POSIX.
5955
5956           Portability issues: "setpriority" in perlport.
5957
5958       setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5959           Sets the socket option requested.  Returns "undef" on error.  Use
5960           integer constants provided by the "Socket" module for LEVEL and
5961           OPNAME.  Values for LEVEL can also be obtained from getprotobyname.
5962           OPTVAL might either be a packed string or an integer.  An integer
5963           OPTVAL is shorthand for pack("i", OPTVAL).
5964
5965           An example disabling Nagle's algorithm on a socket:
5966
5967               use Socket qw(IPPROTO_TCP TCP_NODELAY);
5968               setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5969
5970           Portability issues: "setsockopt" in perlport.
5971
5972       shift ARRAY
5973       shift
5974           Shifts the first value of the array off and returns it, shortening
5975           the array by 1 and moving everything down.  If there are no
5976           elements in the array, returns the undefined value.  If ARRAY is
5977           omitted, shifts the @_ array within the lexical scope of
5978           subroutines and formats, and the @ARGV array outside a subroutine
5979           and also within the lexical scopes established by the "eval
5980           STRING", "BEGIN {}", "INIT {}", "CHECK {}", "UNITCHECK {}", and
5981           "END {}" constructs.
5982
5983           Starting with Perl 5.14, an experimental feature allowed "shift" to
5984           take a scalar expression. This experiment has been deemed
5985           unsuccessful, and was removed as of Perl 5.24.
5986
5987           See also "unshift", "push", and "pop".  "shift" and "unshift" do
5988           the same thing to the left end of an array that "pop" and "push" do
5989           to the right end.
5990
5991       shmctl ID,CMD,ARG
5992           Calls the System V IPC function shmctl.  You'll probably have to
5993           say
5994
5995               use IPC::SysV;
5996
5997           first to get the correct constant definitions.  If CMD is
5998           "IPC_STAT", then ARG must be a variable that will hold the returned
5999           "shmid_ds" structure.  Returns like ioctl: "undef" for error; "0
6000           but true" for zero; and the actual return value otherwise.  See
6001           also "SysV IPC" in perlipc and the documentation for "IPC::SysV".
6002
6003           Portability issues: "shmctl" in perlport.
6004
6005       shmget KEY,SIZE,FLAGS
6006           Calls the System V IPC function shmget.  Returns the shared memory
6007           segment id, or "undef" on error.  See also "SysV IPC" in perlipc
6008           and the documentation for "IPC::SysV".
6009
6010           Portability issues: "shmget" in perlport.
6011
6012       shmread ID,VAR,POS,SIZE
6013       shmwrite ID,STRING,POS,SIZE
6014           Reads or writes the System V shared memory segment ID starting at
6015           position POS for size SIZE by attaching to it, copying in/out, and
6016           detaching from it.  When reading, VAR must be a variable that will
6017           hold the data read.  When writing, if STRING is too long, only SIZE
6018           bytes are used; if STRING is too short, nulls are written to fill
6019           out SIZE bytes.  Return true if successful, false on error.
6020           "shmread" taints the variable.  See also "SysV IPC" in perlipc and
6021           the documentation for "IPC::SysV" and the "IPC::Shareable" module
6022           from CPAN.
6023
6024           Portability issues: "shmread" in perlport and "shmwrite" in
6025           perlport.
6026
6027       shutdown SOCKET,HOW
6028           Shuts down a socket connection in the manner indicated by HOW,
6029           which has the same interpretation as in the syscall of the same
6030           name.
6031
6032               shutdown($socket, 0);    # I/we have stopped reading data
6033               shutdown($socket, 1);    # I/we have stopped writing data
6034               shutdown($socket, 2);    # I/we have stopped using this socket
6035
6036           This is useful with sockets when you want to tell the other side
6037           you're done writing but not done reading, or vice versa.  It's also
6038           a more insistent form of close because it also disables the file
6039           descriptor in any forked copies in other processes.
6040
6041           Returns 1 for success; on error, returns "undef" if the first
6042           argument is not a valid filehandle, or returns 0 and sets $! for
6043           any other failure.
6044
6045       sin EXPR
6046       sin Returns the sine of EXPR (expressed in radians).  If EXPR is
6047           omitted, returns sine of $_.
6048
6049           For the inverse sine operation, you may use the "Math::Trig::asin"
6050           function, or use this relation:
6051
6052               sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6053
6054       sleep EXPR
6055       sleep
6056           Causes the script to sleep for (integer) EXPR seconds, or forever
6057           if no argument is given.  Returns the integer number of seconds
6058           actually slept.
6059
6060           EXPR should be a positive integer. If called with a negative
6061           integer, "sleep" does not sleep but instead emits a warning, sets
6062           $! ("errno"), and returns zero.
6063
6064           "sleep 0" is permitted, but a function call to the underlying
6065           platform implementation still occurs, with any side effects that
6066           may have.  "sleep 0" is therefore not exactly identical to not
6067           sleeping at all.
6068
6069           May be interrupted if the process receives a signal such as
6070           "SIGALRM".
6071
6072               eval {
6073                   local $SIG{ALRM} = sub { die "Alarm!\n" };
6074                   sleep;
6075               };
6076               die $@ unless $@ eq "Alarm!\n";
6077
6078           You probably cannot mix "alarm" and "sleep" calls, because "sleep"
6079           is often implemented using "alarm".
6080
6081           On some older systems, it may sleep up to a full second less than
6082           what you requested, depending on how it counts seconds.  Most
6083           modern systems always sleep the full amount.  They may appear to
6084           sleep longer than that, however, because your process might not be
6085           scheduled right away in a busy multitasking system.
6086
6087           For delays of finer granularity than one second, the Time::HiRes
6088           module (from CPAN, and starting from Perl 5.8 part of the standard
6089           distribution) provides "usleep".  You may also use Perl's four-
6090           argument version of "select" leaving the first three arguments
6091           undefined, or you might be able to use the "syscall" interface to
6092           access setitimer(2) if your system supports it.  See perlfaq8 for
6093           details.
6094
6095           See also the POSIX module's "pause" function.
6096
6097       socket SOCKET,DOMAIN,TYPE,PROTOCOL
6098           Opens a socket of the specified kind and attaches it to filehandle
6099           SOCKET.  DOMAIN, TYPE, and PROTOCOL are specified the same as for
6100           the syscall of the same name.  You should "use Socket" first to get
6101           the proper definitions imported.  See the examples in "Sockets:
6102           Client/Server Communication" in perlipc.
6103
6104           On systems that support a close-on-exec flag on files, the flag
6105           will be set for the newly opened file descriptor, as determined by
6106           the value of $^F.  See "$^F" in perlvar.
6107
6108       socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6109           Creates an unnamed pair of sockets in the specified domain, of the
6110           specified type.  DOMAIN, TYPE, and PROTOCOL are specified the same
6111           as for the syscall of the same name.  If unimplemented, raises an
6112           exception.  Returns true if successful.
6113
6114           On systems that support a close-on-exec flag on files, the flag
6115           will be set for the newly opened file descriptors, as determined by
6116           the value of $^F.  See "$^F" in perlvar.
6117
6118           Some systems define "pipe" in terms of "socketpair", in which a
6119           call to "pipe($rdr, $wtr)" is essentially:
6120
6121               use Socket;
6122               socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6123               shutdown($rdr, 1);        # no more writing for reader
6124               shutdown($wtr, 0);        # no more reading for writer
6125
6126           See perlipc for an example of socketpair use.  Perl 5.8 and later
6127           will emulate socketpair using IP sockets to localhost if your
6128           system implements sockets but not socketpair.
6129
6130           Portability issues: "socketpair" in perlport.
6131
6132       sort SUBNAME LIST
6133       sort BLOCK LIST
6134       sort LIST
6135           In list context, this sorts the LIST and returns the sorted list
6136           value.  In scalar context, the behaviour of "sort" is undefined.
6137
6138           If SUBNAME or BLOCK is omitted, "sort"s in standard string
6139           comparison order.  If SUBNAME is specified, it gives the name of a
6140           subroutine that returns an integer less than, equal to, or greater
6141           than 0, depending on how the elements of the list are to be
6142           ordered.  (The "<=>" and "cmp" operators are extremely useful in
6143           such routines.)  SUBNAME may be a scalar variable name
6144           (unsubscripted), in which case the value provides the name of (or a
6145           reference to) the actual subroutine to use.  In place of a SUBNAME,
6146           you can provide a BLOCK as an anonymous, in-line sort subroutine.
6147
6148           If the subroutine's prototype is "($$)", the elements to be
6149           compared are passed by reference in @_, as for a normal subroutine.
6150           This is slower than unprototyped subroutines, where the elements to
6151           be compared are passed into the subroutine as the package global
6152           variables $a and $b (see example below).
6153
6154           If the subroutine is an XSUB, the elements to be compared are
6155           pushed on to the stack, the way arguments are usually passed to
6156           XSUBs.  $a and $b are not set.
6157
6158           The values to be compared are always passed by reference and should
6159           not be modified.
6160
6161           You also cannot exit out of the sort block or subroutine using any
6162           of the loop control operators described in perlsyn or with "goto".
6163
6164           When "use locale" (but not "use locale ':not_characters'") is in
6165           effect, "sort LIST" sorts LIST according to the current collation
6166           locale.  See perllocale.
6167
6168           "sort" returns aliases into the original list, much as a for loop's
6169           index variable aliases the list elements.  That is, modifying an
6170           element of a list returned by "sort" (for example, in a "foreach",
6171           "map" or "grep") actually modifies the element in the original
6172           list.  This is usually something to be avoided when writing clear
6173           code.
6174
6175           Historically Perl has varied in whether sorting is stable by
6176           default.  If stability matters, it can be controlled explicitly by
6177           using the sort pragma.
6178
6179           Examples:
6180
6181               # sort lexically
6182               my @articles = sort @files;
6183
6184               # same thing, but with explicit sort routine
6185               my @articles = sort {$a cmp $b} @files;
6186
6187               # now case-insensitively
6188               my @articles = sort {fc($a) cmp fc($b)} @files;
6189
6190               # same thing in reversed order
6191               my @articles = sort {$b cmp $a} @files;
6192
6193               # sort numerically ascending
6194               my @articles = sort {$a <=> $b} @files;
6195
6196               # sort numerically descending
6197               my @articles = sort {$b <=> $a} @files;
6198
6199               # this sorts the %age hash by value instead of key
6200               # using an in-line function
6201               my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6202
6203               # sort using explicit subroutine name
6204               sub byage {
6205                   $age{$a} <=> $age{$b};  # presuming numeric
6206               }
6207               my @sortedclass = sort byage @class;
6208
6209               sub backwards { $b cmp $a }
6210               my @harry  = qw(dog cat x Cain Abel);
6211               my @george = qw(gone chased yz Punished Axed);
6212               print sort @harry;
6213                   # prints AbelCaincatdogx
6214               print sort backwards @harry;
6215                   # prints xdogcatCainAbel
6216               print sort @george, 'to', @harry;
6217                   # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6218
6219               # inefficiently sort by descending numeric compare using
6220               # the first integer after the first = sign, or the
6221               # whole record case-insensitively otherwise
6222
6223               my @new = sort {
6224                   ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6225                                       ||
6226                               fc($a)  cmp  fc($b)
6227               } @old;
6228
6229               # same thing, but much more efficiently;
6230               # we'll build auxiliary indices instead
6231               # for speed
6232               my (@nums, @caps);
6233               for (@old) {
6234                   push @nums, ( /=(\d+)/ ? $1 : undef );
6235                   push @caps, fc($_);
6236               }
6237
6238               my @new = @old[ sort {
6239                                      $nums[$b] <=> $nums[$a]
6240                                               ||
6241                                      $caps[$a] cmp $caps[$b]
6242                                    } 0..$#old
6243                             ];
6244
6245               # same thing, but without any temps
6246               my @new = map { $_->[0] }
6247                      sort { $b->[1] <=> $a->[1]
6248                                      ||
6249                             $a->[2] cmp $b->[2]
6250                      } map { [$_, /=(\d+)/, fc($_)] } @old;
6251
6252               # using a prototype allows you to use any comparison subroutine
6253               # as a sort subroutine (including other package's subroutines)
6254               package Other;
6255               sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are
6256                                                        # not set here
6257               package main;
6258               my @new = sort Other::backwards @old;
6259
6260               # guarantee stability
6261               use sort 'stable';
6262               my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6263
6264           Warning: syntactical care is required when sorting the list
6265           returned from a function.  If you want to sort the list returned by
6266           the function call "find_records(@key)", you can use:
6267
6268               my @contact = sort { $a cmp $b } find_records @key;
6269               my @contact = sort +find_records(@key);
6270               my @contact = sort &find_records(@key);
6271               my @contact = sort(find_records(@key));
6272
6273           If instead you want to sort the array @key with the comparison
6274           routine "find_records()" then you can use:
6275
6276               my @contact = sort { find_records() } @key;
6277               my @contact = sort find_records(@key);
6278               my @contact = sort(find_records @key);
6279               my @contact = sort(find_records (@key));
6280
6281           $a and $b are set as package globals in the package the sort() is
6282           called from.  That means $main::a and $main::b (or $::a and $::b)
6283           in the "main" package, $FooPack::a and $FooPack::b in the "FooPack"
6284           package, etc.  If the sort block is in scope of a "my" or "state"
6285           declaration of $a and/or $b, you must spell out the full name of
6286           the variables in the sort block :
6287
6288              package main;
6289              my $a = "C"; # DANGER, Will Robinson, DANGER !!!
6290
6291              print sort { $a cmp $b }               qw(A C E G B D F H);
6292                                                     # WRONG
6293              sub badlexi { $a cmp $b }
6294              print sort badlexi                     qw(A C E G B D F H);
6295                                                     # WRONG
6296              # the above prints BACFEDGH or some other incorrect ordering
6297
6298              print sort { $::a cmp $::b }           qw(A C E G B D F H);
6299                                                     # OK
6300              print sort { our $a cmp our $b }       qw(A C E G B D F H);
6301                                                     # also OK
6302              print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
6303                                                     # also OK
6304              sub lexi { our $a cmp our $b }
6305              print sort lexi                        qw(A C E G B D F H);
6306                                                     # also OK
6307              # the above print ABCDEFGH
6308
6309           With proper care you may mix package and my (or state) $a and/or
6310           $b:
6311
6312              my $a = {
6313                 tiny   => -2,
6314                 small  => -1,
6315                 normal => 0,
6316                 big    => 1,
6317                 huge   => 2
6318              };
6319
6320              say sort { $a->{our $a} <=> $a->{our $b} }
6321                  qw{ huge normal tiny small big};
6322
6323              # prints tinysmallnormalbighuge
6324
6325           $a and $b are implicitly local to the sort() execution and regain
6326           their former values upon completing the sort.
6327
6328           Sort subroutines written using $a and $b are bound to their calling
6329           package. It is possible, but of limited interest, to define them in
6330           a different package, since the subroutine must still refer to the
6331           calling package's $a and $b :
6332
6333              package Foo;
6334              sub lexi { $Bar::a cmp $Bar::b }
6335              package Bar;
6336              ... sort Foo::lexi ...
6337
6338           Use the prototyped versions (see above) for a more generic
6339           alternative.
6340
6341           The comparison function is required to behave.  If it returns
6342           inconsistent results (sometimes saying $x[1] is less than $x[2] and
6343           sometimes saying the opposite, for example) the results are not
6344           well-defined.
6345
6346           Because "<=>" returns "undef" when either operand is "NaN" (not-a-
6347           number), be careful when sorting with a comparison function like
6348           "$a <=> $b" any lists that might contain a "NaN".  The following
6349           example takes advantage that "NaN != NaN" to eliminate any "NaN"s
6350           from the input list.
6351
6352               my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6353
6354           In this version of perl, the "sort" function is implemented via the
6355           mergesort algorithm.
6356
6357       splice ARRAY,OFFSET,LENGTH,LIST
6358       splice ARRAY,OFFSET,LENGTH
6359       splice ARRAY,OFFSET
6360       splice ARRAY
6361           Removes the elements designated by OFFSET and LENGTH from an array,
6362           and replaces them with the elements of LIST, if any.  In list
6363           context, returns the elements removed from the array.  In scalar
6364           context, returns the last element removed, or "undef" if no
6365           elements are removed.  The array grows or shrinks as necessary.  If
6366           OFFSET is negative then it starts that far from the end of the
6367           array.  If LENGTH is omitted, removes everything from OFFSET
6368           onward.  If LENGTH is negative, removes the elements from OFFSET
6369           onward except for -LENGTH elements at the end of the array.  If
6370           both OFFSET and LENGTH are omitted, removes everything.  If OFFSET
6371           is past the end of the array and a LENGTH was provided, Perl issues
6372           a warning, and splices at the end of the array.
6373
6374           The following equivalences hold (assuming "$#a >= $i" )
6375
6376               push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
6377               pop(@a)             splice(@a,-1)
6378               shift(@a)           splice(@a,0,1)
6379               unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
6380               $a[$i] = $y         splice(@a,$i,1,$y)
6381
6382           "splice" can be used, for example, to implement n-ary queue
6383           processing:
6384
6385               sub nary_print {
6386                 my $n = shift;
6387                 while (my @next_n = splice @_, 0, $n) {
6388                   say join q{ -- }, @next_n;
6389                 }
6390               }
6391
6392               nary_print(3, qw(a b c d e f g h));
6393               # prints:
6394               #   a -- b -- c
6395               #   d -- e -- f
6396               #   g -- h
6397
6398           Starting with Perl 5.14, an experimental feature allowed "splice"
6399           to take a scalar expression. This experiment has been deemed
6400           unsuccessful, and was removed as of Perl 5.24.
6401
6402       split /PATTERN/,EXPR,LIMIT
6403       split /PATTERN/,EXPR
6404       split /PATTERN/
6405       split
6406           Splits the string EXPR into a list of strings and returns the list
6407           in list context, or the size of the list in scalar context.  (Prior
6408           to Perl 5.11, it also overwrote @_ with the list in void and scalar
6409           context. If you target old perls, beware.)
6410
6411           If only PATTERN is given, EXPR defaults to $_.
6412
6413           Anything in EXPR that matches PATTERN is taken to be a separator
6414           that separates the EXPR into substrings (called "fields") that do
6415           not include the separator.  Note that a separator may be longer
6416           than one character or even have no characters at all (the empty
6417           string, which is a zero-width match).
6418
6419           The PATTERN need not be constant; an expression may be used to
6420           specify a pattern that varies at runtime.
6421
6422           If PATTERN matches the empty string, the EXPR is split at the match
6423           position (between characters).  As an example, the following:
6424
6425               my @x = split(/b/, "abc"); # ("a", "c")
6426
6427           uses the "b" in 'abc' as a separator to produce the list ("a",
6428           "c").  However, this:
6429
6430               my @x = split(//, "abc"); # ("a", "b", "c")
6431
6432           uses empty string matches as separators; thus, the empty string may
6433           be used to split EXPR into a list of its component characters.
6434
6435           As a special case for "split", the empty pattern given in match
6436           operator syntax ("//") specifically matches the empty string, which
6437           is contrary to its usual interpretation as the last successful
6438           match.
6439
6440           If PATTERN is "/^/", then it is treated as if it used the multiline
6441           modifier ("/^/m"), since it isn't much use otherwise.
6442
6443           "/m" and any of the other pattern modifiers valid for "qr"
6444           (summarized in "qr/STRING/msixpodualn" in perlop) may be specified
6445           explicitly.
6446
6447           As another special case, "split" emulates the default behavior of
6448           the command line tool awk when the PATTERN is either omitted or a
6449           string composed of a single space character (such as ' ' or "\x20",
6450           but not e.g. "/ /").  In this case, any leading whitespace in EXPR
6451           is removed before splitting occurs, and the PATTERN is instead
6452           treated as if it were "/\s+/"; in particular, this means that any
6453           contiguous whitespace (not just a single space character) is used
6454           as a separator.
6455
6456               my @x = split(" ", "  Quick brown fox\n");
6457               # ("Quick", "brown", "fox")
6458
6459               my @x = split(" ", "RED\tGREEN\tBLUE");
6460               # ("RED", "GREEN", "BLUE")
6461
6462           Using split in this fashion is very similar to how "qw//" works.
6463
6464           However, this special treatment can be avoided by specifying the
6465           pattern "/ /" instead of the string " ", thereby allowing only a
6466           single space character to be a separator.  In earlier Perls this
6467           special case was restricted to the use of a plain " " as the
6468           pattern argument to split; in Perl 5.18.0 and later this special
6469           case is triggered by any expression which evaluates to the simple
6470           string " ".
6471
6472           As of Perl 5.28, this special-cased whitespace splitting works as
6473           expected in the scope of "use feature 'unicode_strings'". In
6474           previous versions, and outside the scope of that feature, it
6475           exhibits "The "Unicode Bug"" in perlunicode: characters that are
6476           whitespace according to Unicode rules but not according to ASCII
6477           rules can be treated as part of fields rather than as field
6478           separators, depending on the string's internal encoding.
6479
6480           If omitted, PATTERN defaults to a single space, " ", triggering the
6481           previously described awk emulation.
6482
6483           If LIMIT is specified and positive, it represents the maximum
6484           number of fields into which the EXPR may be split; in other words,
6485           LIMIT is one greater than the maximum number of times EXPR may be
6486           split.  Thus, the LIMIT value 1 means that EXPR may be split a
6487           maximum of zero times, producing a maximum of one field (namely,
6488           the entire value of EXPR).  For instance:
6489
6490               my @x = split(//, "abc", 1); # ("abc")
6491               my @x = split(//, "abc", 2); # ("a", "bc")
6492               my @x = split(//, "abc", 3); # ("a", "b", "c")
6493               my @x = split(//, "abc", 4); # ("a", "b", "c")
6494
6495           If LIMIT is negative, it is treated as if it were instead
6496           arbitrarily large; as many fields as possible are produced.
6497
6498           If LIMIT is omitted (or, equivalently, zero), then it is usually
6499           treated as if it were instead negative but with the exception that
6500           trailing empty fields are stripped (empty leading fields are always
6501           preserved); if all fields are empty, then all fields are considered
6502           to be trailing (and are thus stripped in this case).  Thus, the
6503           following:
6504
6505               my @x = split(/,/, "a,b,c,,,"); # ("a", "b", "c")
6506
6507           produces only a three element list.
6508
6509               my @x = split(/,/, "a,b,c,,,", -1); # ("a", "b", "c", "", "", "")
6510
6511           produces a six element list.
6512
6513           In time-critical applications, it is worthwhile to avoid splitting
6514           into more fields than necessary.  Thus, when assigning to a list,
6515           if LIMIT is omitted (or zero), then LIMIT is treated as though it
6516           were one larger than the number of variables in the list; for the
6517           following, LIMIT is implicitly 3:
6518
6519               my ($login, $passwd) = split(/:/);
6520
6521           Note that splitting an EXPR that evaluates to the empty string
6522           always produces zero fields, regardless of the LIMIT specified.
6523
6524           An empty leading field is produced when there is a positive-width
6525           match at the beginning of EXPR.  For instance:
6526
6527               my @x = split(/ /, " abc"); # ("", "abc")
6528
6529           splits into two elements.  However, a zero-width match at the
6530           beginning of EXPR never produces an empty field, so that:
6531
6532               my @x = split(//, " abc"); # (" ", "a", "b", "c")
6533
6534           splits into four elements instead of five.
6535
6536           An empty trailing field, on the other hand, is produced when there
6537           is a match at the end of EXPR, regardless of the length of the
6538           match (of course, unless a non-zero LIMIT is given explicitly, such
6539           fields are removed, as in the last example).  Thus:
6540
6541               my @x = split(//, " abc", -1); # (" ", "a", "b", "c", "")
6542
6543           If the PATTERN contains capturing groups, then for each separator,
6544           an additional field is produced for each substring captured by a
6545           group (in the order in which the groups are specified, as per
6546           backreferences); if any group does not match, then it captures the
6547           "undef" value instead of a substring.  Also, note that any such
6548           additional field is produced whenever there is a separator (that
6549           is, whenever a split occurs), and such an additional field does not
6550           count towards the LIMIT.  Consider the following expressions
6551           evaluated in list context (each returned list is provided in the
6552           associated comment):
6553
6554               my @x = split(/-|,/    , "1-10,20", 3);
6555               # ("1", "10", "20")
6556
6557               my @x = split(/(-|,)/  , "1-10,20", 3);
6558               # ("1", "-", "10", ",", "20")
6559
6560               my @x = split(/-|(,)/  , "1-10,20", 3);
6561               # ("1", undef, "10", ",", "20")
6562
6563               my @x = split(/(-)|,/  , "1-10,20", 3);
6564               # ("1", "-", "10", undef, "20")
6565
6566               my @x = split(/(-)|(,)/, "1-10,20", 3);
6567               # ("1", "-", undef, "10", undef, ",", "20")
6568
6569       sprintf FORMAT, LIST
6570           Returns a string formatted by the usual "printf" conventions of the
6571           C library function "sprintf".  See below for more details and see
6572           sprintf(3) or printf(3) on your system for an explanation of the
6573           general principles.
6574
6575           For example:
6576
6577                   # Format number with up to 8 leading zeroes
6578                   my $result = sprintf("%08d", $number);
6579
6580                   # Round number to 3 digits after decimal point
6581                   my $rounded = sprintf("%.3f", $number);
6582
6583           Perl does its own "sprintf" formatting: it emulates the C function
6584           sprintf(3), but doesn't use it except for floating-point numbers,
6585           and even then only standard modifiers are allowed.  Non-standard
6586           extensions in your local sprintf(3) are therefore unavailable from
6587           Perl.
6588
6589           Unlike "printf", "sprintf" does not do what you probably mean when
6590           you pass it an array as your first argument.  The array is given
6591           scalar context, and instead of using the 0th element of the array
6592           as the format, Perl will use the count of elements in the array as
6593           the format, which is almost never useful.
6594
6595           Perl's "sprintf" permits the following universally-known
6596           conversions:
6597
6598              %%    a percent sign
6599              %c    a character with the given number
6600              %s    a string
6601              %d    a signed integer, in decimal
6602              %u    an unsigned integer, in decimal
6603              %o    an unsigned integer, in octal
6604              %x    an unsigned integer, in hexadecimal
6605              %e    a floating-point number, in scientific notation
6606              %f    a floating-point number, in fixed decimal notation
6607              %g    a floating-point number, in %e or %f notation
6608
6609           In addition, Perl permits the following widely-supported
6610           conversions:
6611
6612              %X    like %x, but using upper-case letters
6613              %E    like %e, but using an upper-case "E"
6614              %G    like %g, but with an upper-case "E" (if applicable)
6615              %b    an unsigned integer, in binary
6616              %B    like %b, but using an upper-case "B" with the # flag
6617              %p    a pointer (outputs the Perl value's address in hexadecimal)
6618              %n    special: *stores* the number of characters output so far
6619                    into the next argument in the parameter list
6620              %a    hexadecimal floating point
6621              %A    like %a, but using upper-case letters
6622
6623           Finally, for backward (and we do mean "backward") compatibility,
6624           Perl permits these unnecessary but widely-supported conversions:
6625
6626              %i    a synonym for %d
6627              %D    a synonym for %ld
6628              %U    a synonym for %lu
6629              %O    a synonym for %lo
6630              %F    a synonym for %f
6631
6632           Note that the number of exponent digits in the scientific notation
6633           produced by %e, %E, %g and %G for numbers with the modulus of the
6634           exponent less than 100 is system-dependent: it may be three or less
6635           (zero-padded as necessary).  In other words, 1.23 times ten to the
6636           99th may be either "1.23e99" or "1.23e099".  Similarly for %a and
6637           %A: the exponent or the hexadecimal digits may float: especially
6638           the "long doubles" Perl configuration option may cause surprises.
6639
6640           Between the "%" and the format letter, you may specify several
6641           additional attributes controlling the interpretation of the format.
6642           In order, these are:
6643
6644           format parameter index
6645               An explicit format parameter index, such as "2$".  By default
6646               sprintf will format the next unused argument in the list, but
6647               this allows you to take the arguments out of order:
6648
6649                 printf '%2$d %1$d', 12, 34;      # prints "34 12"
6650                 printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"
6651
6652           flags
6653               one or more of:
6654
6655                  space   prefix non-negative number with a space
6656                  +       prefix non-negative number with a plus sign
6657                  -       left-justify within the field
6658                  0       use zeros, not spaces, to right-justify
6659                  #       ensure the leading "0" for any octal,
6660                          prefix non-zero hexadecimal with "0x" or "0X",
6661                          prefix non-zero binary with "0b" or "0B"
6662
6663               For example:
6664
6665                 printf '<% d>',  12;   # prints "< 12>"
6666                 printf '<% d>',   0;   # prints "< 0>"
6667                 printf '<% d>', -12;   # prints "<-12>"
6668                 printf '<%+d>',  12;   # prints "<+12>"
6669                 printf '<%+d>',   0;   # prints "<+0>"
6670                 printf '<%+d>', -12;   # prints "<-12>"
6671                 printf '<%6s>',  12;   # prints "<    12>"
6672                 printf '<%-6s>', 12;   # prints "<12    >"
6673                 printf '<%06s>', 12;   # prints "<000012>"
6674                 printf '<%#o>',  12;   # prints "<014>"
6675                 printf '<%#x>',  12;   # prints "<0xc>"
6676                 printf '<%#X>',  12;   # prints "<0XC>"
6677                 printf '<%#b>',  12;   # prints "<0b1100>"
6678                 printf '<%#B>',  12;   # prints "<0B1100>"
6679
6680               When a space and a plus sign are given as the flags at once,
6681               the space is ignored.
6682
6683                 printf '<%+ d>', 12;   # prints "<+12>"
6684                 printf '<% +d>', 12;   # prints "<+12>"
6685
6686               When the # flag and a precision are given in the %o conversion,
6687               the precision is incremented if it's necessary for the leading
6688               "0".
6689
6690                 printf '<%#.5o>', 012;      # prints "<00012>"
6691                 printf '<%#.5o>', 012345;   # prints "<012345>"
6692                 printf '<%#.0o>', 0;        # prints "<0>"
6693
6694           vector flag
6695               This flag tells Perl to interpret the supplied string as a
6696               vector of integers, one for each character in the string.  Perl
6697               applies the format to each integer in turn, then joins the
6698               resulting strings with a separator (a dot "." by default).
6699               This can be useful for displaying ordinal values of characters
6700               in arbitrary strings:
6701
6702                 printf "%vd", "AB\x{100}";           # prints "65.66.256"
6703                 printf "version is v%vd\n", $^V;     # Perl's version
6704
6705               Put an asterisk "*" before the "v" to override the string to
6706               use to separate the numbers:
6707
6708                 printf "address is %*vX\n", ":", $addr;   # IPv6 address
6709                 printf "bits are %0*v8b\n", " ", $bits;   # random bitstring
6710
6711               You can also explicitly specify the argument number to use for
6712               the join string using something like "*2$v"; for example:
6713
6714                 printf '%*4$vX %*4$vX %*4$vX',       # 3 IPv6 addresses
6715                         @addr[1..3], ":";
6716
6717           (minimum) width
6718               Arguments are usually formatted to be only as wide as required
6719               to display the given value.  You can override the width by
6720               putting a number here, or get the width from the next argument
6721               (with "*") or from a specified argument (e.g., with "*2$"):
6722
6723                printf "<%s>", "a";       # prints "<a>"
6724                printf "<%6s>", "a";      # prints "<     a>"
6725                printf "<%*s>", 6, "a";   # prints "<     a>"
6726                printf '<%*2$s>', "a", 6; # prints "<     a>"
6727                printf "<%2s>", "long";   # prints "<long>" (does not truncate)
6728
6729               If a field width obtained through "*" is negative, it has the
6730               same effect as the "-" flag: left-justification.
6731
6732           precision, or maximum width
6733               You can specify a precision (for numeric conversions) or a
6734               maximum width (for string conversions) by specifying a "."
6735               followed by a number.  For floating-point formats except "g"
6736               and "G", this specifies how many places right of the decimal
6737               point to show (the default being 6).  For example:
6738
6739                 # these examples are subject to system-specific variation
6740                 printf '<%f>', 1;    # prints "<1.000000>"
6741                 printf '<%.1f>', 1;  # prints "<1.0>"
6742                 printf '<%.0f>', 1;  # prints "<1>"
6743                 printf '<%e>', 10;   # prints "<1.000000e+01>"
6744                 printf '<%.1e>', 10; # prints "<1.0e+01>"
6745
6746               For "g" and "G", this specifies the maximum number of
6747               significant digits to show; for example:
6748
6749                 # These examples are subject to system-specific variation.
6750                 printf '<%g>', 1;        # prints "<1>"
6751                 printf '<%.10g>', 1;     # prints "<1>"
6752                 printf '<%g>', 100;      # prints "<100>"
6753                 printf '<%.1g>', 100;    # prints "<1e+02>"
6754                 printf '<%.2g>', 100.01; # prints "<1e+02>"
6755                 printf '<%.5g>', 100.01; # prints "<100.01>"
6756                 printf '<%.4g>', 100.01; # prints "<100>"
6757                 printf '<%.1g>', 0.0111; # prints "<0.01>"
6758                 printf '<%.2g>', 0.0111; # prints "<0.011>"
6759                 printf '<%.3g>', 0.0111; # prints "<0.0111>"
6760
6761               For integer conversions, specifying a precision implies that
6762               the output of the number itself should be zero-padded to this
6763               width, where the 0 flag is ignored:
6764
6765                 printf '<%.6d>', 1;      # prints "<000001>"
6766                 printf '<%+.6d>', 1;     # prints "<+000001>"
6767                 printf '<%-10.6d>', 1;   # prints "<000001    >"
6768                 printf '<%10.6d>', 1;    # prints "<    000001>"
6769                 printf '<%010.6d>', 1;   # prints "<    000001>"
6770                 printf '<%+10.6d>', 1;   # prints "<   +000001>"
6771
6772                 printf '<%.6x>', 1;      # prints "<000001>"
6773                 printf '<%#.6x>', 1;     # prints "<0x000001>"
6774                 printf '<%-10.6x>', 1;   # prints "<000001    >"
6775                 printf '<%10.6x>', 1;    # prints "<    000001>"
6776                 printf '<%010.6x>', 1;   # prints "<    000001>"
6777                 printf '<%#10.6x>', 1;   # prints "<  0x000001>"
6778
6779               For string conversions, specifying a precision truncates the
6780               string to fit the specified width:
6781
6782                 printf '<%.5s>', "truncated";   # prints "<trunc>"
6783                 printf '<%10.5s>', "truncated"; # prints "<     trunc>"
6784
6785               You can also get the precision from the next argument using
6786               ".*", or from a specified argument (e.g., with ".*2$"):
6787
6788                 printf '<%.6x>', 1;       # prints "<000001>"
6789                 printf '<%.*x>', 6, 1;    # prints "<000001>"
6790
6791                 printf '<%.*2$x>', 1, 6;  # prints "<000001>"
6792
6793                 printf '<%6.*2$x>', 1, 4; # prints "<  0001>"
6794
6795               If a precision obtained through "*" is negative, it counts as
6796               having no precision at all.
6797
6798                 printf '<%.*s>',  7, "string";   # prints "<string>"
6799                 printf '<%.*s>',  3, "string";   # prints "<str>"
6800                 printf '<%.*s>',  0, "string";   # prints "<>"
6801                 printf '<%.*s>', -1, "string";   # prints "<string>"
6802
6803                 printf '<%.*d>',  1, 0;   # prints "<0>"
6804                 printf '<%.*d>',  0, 0;   # prints "<>"
6805                 printf '<%.*d>', -1, 0;   # prints "<0>"
6806
6807           size
6808               For numeric conversions, you can specify the size to interpret
6809               the number as using "l", "h", "V", "q", "L", or "ll".  For
6810               integer conversions ("d u o x X b i D U O"), numbers are
6811               usually assumed to be whatever the default integer size is on
6812               your platform (usually 32 or 64 bits), but you can override
6813               this to use instead one of the standard C types, as supported
6814               by the compiler used to build Perl:
6815
6816                  hh          interpret integer as C type "char" or "unsigned
6817                              char" on Perl 5.14 or later
6818                  h           interpret integer as C type "short" or
6819                              "unsigned short"
6820                  j           interpret integer as C type "intmax_t" on Perl
6821                              5.14 or later; and prior to Perl 5.30, only with
6822                              a C99 compiler (unportable)
6823                  l           interpret integer as C type "long" or
6824                              "unsigned long"
6825                  q, L, or ll interpret integer as C type "long long",
6826                              "unsigned long long", or "quad" (typically
6827                              64-bit integers)
6828                  t           interpret integer as C type "ptrdiff_t" on Perl
6829                              5.14 or later
6830                  z           interpret integer as C types "size_t" or
6831                              "ssize_t" on Perl 5.14 or later
6832
6833               Note that, in general, using the "l" modifier (for example,
6834               when writing "%ld" or "%lu" instead of "%d" and "%u") is
6835               unnecessary when used from Perl code.  Moreover, it may be
6836               harmful, for example on Windows 64-bit where a long is 32-bits.
6837
6838               As of 5.14, none of these raises an exception if they are not
6839               supported on your platform.  However, if warnings are enabled,
6840               a warning of the "printf" warning class is issued on an
6841               unsupported conversion flag.  Should you instead prefer an
6842               exception, do this:
6843
6844                   use warnings FATAL => "printf";
6845
6846               If you would like to know about a version dependency before you
6847               start running the program, put something like this at its top:
6848
6849                   use 5.014;  # for hh/j/t/z/ printf modifiers
6850
6851               You can find out whether your Perl supports quads via Config:
6852
6853                   use Config;
6854                   if ($Config{use64bitint} eq "define"
6855                       || $Config{longsize} >= 8) {
6856                       print "Nice quads!\n";
6857                   }
6858
6859               For floating-point conversions ("e f g E F G"), numbers are
6860               usually assumed to be the default floating-point size on your
6861               platform (double or long double), but you can force "long
6862               double" with "q", "L", or "ll" if your platform supports them.
6863               You can find out whether your Perl supports long doubles via
6864               Config:
6865
6866                   use Config;
6867                   print "long doubles\n" if $Config{d_longdbl} eq "define";
6868
6869               You can find out whether Perl considers "long double" to be the
6870               default floating-point size to use on your platform via Config:
6871
6872                   use Config;
6873                   if ($Config{uselongdouble} eq "define") {
6874                       print "long doubles by default\n";
6875                   }
6876
6877               It can also be that long doubles and doubles are the same
6878               thing:
6879
6880                       use Config;
6881                       ($Config{doublesize} == $Config{longdblsize}) &&
6882                               print "doubles are long doubles\n";
6883
6884               The size specifier "V" has no effect for Perl code, but is
6885               supported for compatibility with XS code.  It means "use the
6886               standard size for a Perl integer or floating-point number",
6887               which is the default.
6888
6889           order of arguments
6890               Normally, "sprintf" takes the next unused argument as the value
6891               to format for each format specification.  If the format
6892               specification uses "*" to require additional arguments, these
6893               are consumed from the argument list in the order they appear in
6894               the format specification before the value to format.  Where an
6895               argument is specified by an explicit index, this does not
6896               affect the normal order for the arguments, even when the
6897               explicitly specified index would have been the next argument.
6898
6899               So:
6900
6901                   printf "<%*.*s>", $a, $b, $c;
6902
6903               uses $a for the width, $b for the precision, and $c as the
6904               value to format; while:
6905
6906                 printf '<%*1$.*s>', $a, $b;
6907
6908               would use $a for the width and precision, and $b as the value
6909               to format.
6910
6911               Here are some more examples; be aware that when using an
6912               explicit index, the "$" may need escaping:
6913
6914                printf "%2\$d %d\n",      12, 34;     # will print "34 12\n"
6915                printf "%2\$d %d %d\n",   12, 34;     # will print "34 12 34\n"
6916                printf "%3\$d %d %d\n",   12, 34, 56; # will print "56 12 34\n"
6917                printf "%2\$*3\$d %d\n",  12, 34,  3; # will print " 34 12\n"
6918                printf "%*1\$.*f\n",       4,  5, 10; # will print "5.0000\n"
6919
6920           If "use locale" (including "use locale ':not_characters'") is in
6921           effect and "POSIX::setlocale" has been called, the character used
6922           for the decimal separator in formatted floating-point numbers is
6923           affected by the "LC_NUMERIC" locale.  See perllocale and POSIX.
6924
6925       sqrt EXPR
6926       sqrt
6927           Return the positive square root of EXPR.  If EXPR is omitted, uses
6928           $_.  Works only for non-negative operands unless you've loaded the
6929           "Math::Complex" module.
6930
6931               use Math::Complex;
6932               print sqrt(-4);    # prints 2i
6933
6934       srand EXPR
6935       srand
6936           Sets and returns the random number seed for the "rand" operator.
6937
6938           The point of the function is to "seed" the "rand" function so that
6939           "rand" can produce a different sequence each time you run your
6940           program.  When called with a parameter, "srand" uses that for the
6941           seed; otherwise it (semi-)randomly chooses a seed.  In either case,
6942           starting with Perl 5.14, it returns the seed.  To signal that your
6943           code will work only on Perls of a recent vintage:
6944
6945               use 5.014;  # so srand returns the seed
6946
6947           If "srand" is not called explicitly, it is called implicitly
6948           without a parameter at the first use of the "rand" operator.
6949           However, there are a few situations where programs are likely to
6950           want to call "srand".  One is for generating predictable results,
6951           generally for testing or debugging.  There, you use "srand($seed)",
6952           with the same $seed each time.  Another case is that you may want
6953           to call "srand" after a "fork" to avoid child processes sharing the
6954           same seed value as the parent (and consequently each other).
6955
6956           Do not call "srand()" (i.e., without an argument) more than once
6957           per process.  The internal state of the random number generator
6958           should contain more entropy than can be provided by any seed, so
6959           calling "srand" again actually loses randomness.
6960
6961           Most implementations of "srand" take an integer and will silently
6962           truncate decimal numbers.  This means "srand(42)" will usually
6963           produce the same results as "srand(42.1)".  To be safe, always pass
6964           "srand" an integer.
6965
6966           A typical use of the returned seed is for a test program which has
6967           too many combinations to test comprehensively in the time available
6968           to it each run.  It can test a random subset each time, and should
6969           there be a failure, log the seed used for that run so that it can
6970           later be used to reproduce the same results.
6971
6972           "rand" is not cryptographically secure.  You should not rely on it
6973           in security-sensitive situations.  As of this writing, a number of
6974           third-party CPAN modules offer random number generators intended by
6975           their authors to be cryptographically secure, including:
6976           Data::Entropy, Crypt::Random, Math::Random::Secure, and
6977           Math::TrulyRandom.
6978
6979       stat FILEHANDLE
6980       stat EXPR
6981       stat DIRHANDLE
6982       stat
6983           Returns a 13-element list giving the status info for a file, either
6984           the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR.  If
6985           EXPR is omitted, it stats $_ (not "_"!).  Returns the empty list if
6986           "stat" fails.  Typically used as follows:
6987
6988               my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6989                   $atime,$mtime,$ctime,$blksize,$blocks)
6990                      = stat($filename);
6991
6992           Not all fields are supported on all filesystem types.  Here are the
6993           meanings of the fields:
6994
6995             0 dev      device number of filesystem
6996             1 ino      inode number
6997             2 mode     file mode  (type and permissions)
6998             3 nlink    number of (hard) links to the file
6999             4 uid      numeric user ID of file's owner
7000             5 gid      numeric group ID of file's owner
7001             6 rdev     the device identifier (special files only)
7002             7 size     total size of file, in bytes
7003             8 atime    last access time in seconds since the epoch
7004             9 mtime    last modify time in seconds since the epoch
7005            10 ctime    inode change time in seconds since the epoch (*)
7006            11 blksize  preferred I/O size in bytes for interacting with the
7007                        file (may vary from file to file)
7008            12 blocks   actual number of system-specific blocks allocated
7009                        on disk (often, but not always, 512 bytes each)
7010
7011           (The epoch was at 00:00 January 1, 1970 GMT.)
7012
7013           (*) Not all fields are supported on all filesystem types.  Notably,
7014           the ctime field is non-portable.  In particular, you cannot expect
7015           it to be a "creation time"; see "Files and Filesystems" in perlport
7016           for details.
7017
7018           If "stat" is passed the special filehandle consisting of an
7019           underline, no stat is done, but the current contents of the stat
7020           structure from the last "stat", "lstat", or filetest are returned.
7021           Example:
7022
7023               if (-x $file && (($d) = stat(_)) && $d < 0) {
7024                   print "$file is executable NFS file\n";
7025               }
7026
7027           (This works on machines only for which the device number is
7028           negative under NFS.)
7029
7030           On some platforms inode numbers are of a type larger than perl
7031           knows how to handle as integer numerical values.  If necessary, an
7032           inode number will be returned as a decimal string in order to
7033           preserve the entire value.  If used in a numeric context, this will
7034           be converted to a floating-point numerical value, with rounding, a
7035           fate that is best avoided.  Therefore, you should prefer to compare
7036           inode numbers using "eq" rather than "==".  "eq" will work fine on
7037           inode numbers that are represented numerically, as well as those
7038           represented as strings.
7039
7040           Because the mode contains both the file type and its permissions,
7041           you should mask off the file type portion and (s)printf using a
7042           "%o" if you want to see the real permissions.
7043
7044               my $mode = (stat($filename))[2];
7045               printf "Permissions are %04o\n", $mode & 07777;
7046
7047           In scalar context, "stat" returns a boolean value indicating
7048           success or failure, and, if successful, sets the information
7049           associated with the special filehandle "_".
7050
7051           The File::stat module provides a convenient, by-name access
7052           mechanism:
7053
7054               use File::stat;
7055               my $sb = stat($filename);
7056               printf "File is %s, size is %s, perm %04o, mtime %s\n",
7057                      $filename, $sb->size, $sb->mode & 07777,
7058                      scalar localtime $sb->mtime;
7059
7060           You can import symbolic mode constants ("S_IF*") and functions
7061           ("S_IS*") from the Fcntl module:
7062
7063               use Fcntl ':mode';
7064
7065               my $mode = (stat($filename))[2];
7066
7067               my $user_rwx      = ($mode & S_IRWXU) >> 6;
7068               my $group_read    = ($mode & S_IRGRP) >> 3;
7069               my $other_execute =  $mode & S_IXOTH;
7070
7071               printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7072
7073               my $is_setuid     =  $mode & S_ISUID;
7074               my $is_directory  =  S_ISDIR($mode);
7075
7076           You could write the last two using the "-u" and "-d" operators.
7077           Commonly available "S_IF*" constants are:
7078
7079               # Permissions: read, write, execute, for user, group, others.
7080
7081               S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7082               S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7083               S_IRWXO S_IROTH S_IWOTH S_IXOTH
7084
7085               # Setuid/Setgid/Stickiness/SaveText.
7086               # Note that the exact meaning of these is system-dependent.
7087
7088               S_ISUID S_ISGID S_ISVTX S_ISTXT
7089
7090               # File types.  Not all are necessarily available on
7091               # your system.
7092
7093               S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7094               S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7095
7096               # The following are compatibility aliases for S_IRUSR,
7097               # S_IWUSR, and S_IXUSR.
7098
7099               S_IREAD S_IWRITE S_IEXEC
7100
7101           and the "S_IF*" functions are
7102
7103               S_IMODE($mode)    the part of $mode containing the permission
7104                                 bits and the setuid/setgid/sticky bits
7105
7106               S_IFMT($mode)     the part of $mode containing the file type
7107                                 which can be bit-anded with (for example)
7108                                 S_IFREG or with the following functions
7109
7110               # The operators -f, -d, -l, -b, -c, -p, and -S.
7111
7112               S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7113               S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7114
7115               # No direct -X operator counterpart, but for the first one
7116               # the -g operator is often equivalent.  The ENFMT stands for
7117               # record flocking enforcement, a platform-dependent feature.
7118
7119               S_ISENFMT($mode) S_ISWHT($mode)
7120
7121           See your native chmod(2) and stat(2) documentation for more details
7122           about the "S_*" constants.  To get status info for a symbolic link
7123           instead of the target file behind the link, use the "lstat"
7124           function.
7125
7126           Portability issues: "stat" in perlport.
7127
7128       state VARLIST
7129       state TYPE VARLIST
7130       state VARLIST : ATTRS
7131       state TYPE VARLIST : ATTRS
7132           "state" declares a lexically scoped variable, just like "my".
7133           However, those variables will never be reinitialized, contrary to
7134           lexical variables that are reinitialized each time their enclosing
7135           block is entered.  See "Persistent Private Variables" in perlsub
7136           for details.
7137
7138           If more than one variable is listed, the list must be placed in
7139           parentheses.  With a parenthesised list, "undef" can be used as a
7140           dummy placeholder.  However, since initialization of state
7141           variables in such lists is currently not possible this would serve
7142           no purpose.
7143
7144           Redeclaring a variable in the same scope or statement will "shadow"
7145           the previous declaration, creating a new instance and preventing
7146           access to the previous one. This is usually undesired and, if
7147           warnings are enabled, will result in a warning in the "shadow"
7148           category.
7149
7150           "state" is available only if the "state" feature is enabled or if
7151           it is prefixed with "CORE::".  The "state" feature is enabled
7152           automatically with a "use v5.10" (or higher) declaration in the
7153           current scope.
7154
7155       study SCALAR
7156       study
7157           At this time, "study" does nothing. This may change in the future.
7158
7159           Prior to Perl version 5.16, it would create an inverted index of
7160           all characters that occurred in the given SCALAR (or $_ if
7161           unspecified). When matching a pattern, the rarest character from
7162           the pattern would be looked up in this index. Rarity was based on
7163           some static frequency tables constructed from some C programs and
7164           English text.
7165
7166       sub NAME BLOCK
7167       sub NAME (PROTO) BLOCK
7168       sub NAME : ATTRS BLOCK
7169       sub NAME (PROTO) : ATTRS BLOCK
7170           This is subroutine definition, not a real function per se.  Without
7171           a BLOCK it's just a forward declaration.  Without a NAME, it's an
7172           anonymous function declaration, so does return a value: the CODE
7173           ref of the closure just created.
7174
7175           See perlsub and perlref for details about subroutines and
7176           references; see attributes and Attribute::Handlers for more
7177           information about attributes.
7178
7179       __SUB__
7180           A special token that returns a reference to the current subroutine,
7181           or "undef" outside of a subroutine.
7182
7183           The behaviour of "__SUB__" within a regex code block (such as
7184           "/(?{...})/") is subject to change.
7185
7186           This token is only available under "use v5.16" or the "current_sub"
7187           feature.  See feature.
7188
7189       substr EXPR,OFFSET,LENGTH,REPLACEMENT
7190       substr EXPR,OFFSET,LENGTH
7191       substr EXPR,OFFSET
7192           Extracts a substring out of EXPR and returns it.  First character
7193           is at offset zero.  If OFFSET is negative, starts that far back
7194           from the end of the string.  If LENGTH is omitted, returns
7195           everything through the end of the string.  If LENGTH is negative,
7196           leaves that many characters off the end of the string.
7197
7198               my $s = "The black cat climbed the green tree";
7199               my $color  = substr $s, 4, 5;      # black
7200               my $middle = substr $s, 4, -11;    # black cat climbed the
7201               my $end    = substr $s, 14;        # climbed the green tree
7202               my $tail   = substr $s, -4;        # tree
7203               my $z      = substr $s, -4, 2;     # tr
7204
7205           You can use the "substr" function as an lvalue, in which case EXPR
7206           must itself be an lvalue.  If you assign something shorter than
7207           LENGTH, the string will shrink, and if you assign something longer
7208           than LENGTH, the string will grow to accommodate it.  To keep the
7209           string the same length, you may need to pad or chop your value
7210           using "sprintf".
7211
7212           If OFFSET and LENGTH specify a substring that is partly outside the
7213           string, only the part within the string is returned.  If the
7214           substring is beyond either end of the string, "substr" returns the
7215           undefined value and produces a warning.  When used as an lvalue,
7216           specifying a substring that is entirely outside the string raises
7217           an exception.  Here's an example showing the behavior for boundary
7218           cases:
7219
7220               my $name = 'fred';
7221               substr($name, 4) = 'dy';         # $name is now 'freddy'
7222               my $null = substr $name, 6, 2;   # returns "" (no warning)
7223               my $oops = substr $name, 7;      # returns undef, with warning
7224               substr($name, 7) = 'gap';        # raises an exception
7225
7226           An alternative to using "substr" as an lvalue is to specify the
7227           replacement string as the 4th argument.  This allows you to replace
7228           parts of the EXPR and return what was there before in one
7229           operation, just as you can with "splice".
7230
7231               my $s = "The black cat climbed the green tree";
7232               my $z = substr $s, 14, 7, "jumped from";    # climbed
7233               # $s is now "The black cat jumped from the green tree"
7234
7235           Note that the lvalue returned by the three-argument version of
7236           "substr" acts as a 'magic bullet'; each time it is assigned to, it
7237           remembers which part of the original string is being modified; for
7238           example:
7239
7240               my $x = '1234';
7241               for (substr($x,1,2)) {
7242                   $_ = 'a';   print $x,"\n";    # prints 1a4
7243                   $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
7244                   $x = '56789';
7245                   $_ = 'pq';  print $x,"\n";    # prints 5pq9
7246               }
7247
7248           With negative offsets, it remembers its position from the end of
7249           the string when the target string is modified:
7250
7251               my $x = '1234';
7252               for (substr($x, -3, 2)) {
7253                   $_ = 'a';   print $x,"\n";    # prints 1a4, as above
7254                   $x = 'abcdefg';
7255                   print $_,"\n";                # prints f
7256               }
7257
7258           Prior to Perl version 5.10, the result of using an lvalue multiple
7259           times was unspecified.  Prior to 5.16, the result with negative
7260           offsets was unspecified.
7261
7262       symlink OLDFILE,NEWFILE
7263           Creates a new filename symbolically linked to the old filename.
7264           Returns 1 for success, 0 otherwise.  On systems that don't support
7265           symbolic links, raises an exception.  To check for that, use eval:
7266
7267               my $symlink_exists = eval { symlink("",""); 1 };
7268
7269           Portability issues: "symlink" in perlport.
7270
7271       syscall NUMBER, LIST
7272           Calls the system call specified as the first element of the list,
7273           passing the remaining elements as arguments to the system call.  If
7274           unimplemented, raises an exception.  The arguments are interpreted
7275           as follows: if a given argument is numeric, the argument is passed
7276           as an int.  If not, the pointer to the string value is passed.  You
7277           are responsible to make sure a string is pre-extended long enough
7278           to receive any result that might be written into a string.  You
7279           can't use a string literal (or other read-only string) as an
7280           argument to "syscall" because Perl has to assume that any string
7281           pointer might be written through.  If your integer arguments are
7282           not literals and have never been interpreted in a numeric context,
7283           you may need to add 0 to them to force them to look like numbers.
7284           This emulates the "syswrite" function (or vice versa):
7285
7286               require 'syscall.ph';        # may need to run h2ph
7287               my $s = "hi there\n";
7288               syscall(SYS_write(), fileno(STDOUT), $s, length $s);
7289
7290           Note that Perl supports passing of up to only 14 arguments to your
7291           syscall, which in practice should (usually) suffice.
7292
7293           Syscall returns whatever value returned by the system call it
7294           calls.  If the system call fails, "syscall" returns "-1" and sets
7295           $! (errno).  Note that some system calls can legitimately return
7296           "-1".  The proper way to handle such calls is to assign "$! = 0"
7297           before the call, then check the value of $! if "syscall" returns
7298           "-1".
7299
7300           There's a problem with "syscall(SYS_pipe())": it returns the file
7301           number of the read end of the pipe it creates, but there is no way
7302           to retrieve the file number of the other end.  You can avoid this
7303           problem by using "pipe" instead.
7304
7305           Portability issues: "syscall" in perlport.
7306
7307       sysopen FILEHANDLE,FILENAME,MODE
7308       sysopen FILEHANDLE,FILENAME,MODE,PERMS
7309           Opens the file whose filename is given by FILENAME, and associates
7310           it with FILEHANDLE.  If FILEHANDLE is an expression, its value is
7311           used as the real filehandle wanted; an undefined scalar will be
7312           suitably autovivified.  This function calls the underlying
7313           operating system's open(2) function with the parameters FILENAME,
7314           MODE, and PERMS.
7315
7316           Returns true on success and "undef" otherwise.
7317
7318           PerlIO layers will be applied to the handle the same way they would
7319           in an "open" call that does not specify layers. That is, the
7320           current value of "${^OPEN}" as set by the open pragma in a lexical
7321           scope, or the "-C" commandline option or "PERL_UNICODE" environment
7322           variable in the main program scope, falling back to the platform
7323           defaults as described in "Defaults and how to override them" in
7324           PerlIO. If you want to remove any layers that may transform the
7325           byte stream, use "binmode" after opening it.
7326
7327           The possible values and flag bits of the MODE parameter are system-
7328           dependent; they are available via the standard module "Fcntl".  See
7329           the documentation of your operating system's open(2) syscall to see
7330           which values and flag bits are available.  You may combine several
7331           flags using the "|"-operator.
7332
7333           Some of the most common values are "O_RDONLY" for opening the file
7334           in read-only mode, "O_WRONLY" for opening the file in write-only
7335           mode, and "O_RDWR" for opening the file in read-write mode.
7336
7337           For historical reasons, some values work on almost every system
7338           supported by Perl: 0 means read-only, 1 means write-only, and 2
7339           means read/write.  We know that these values do not work under
7340           OS/390 and on the Macintosh; you probably don't want to use them in
7341           new code.
7342
7343           If the file named by FILENAME does not exist and the "open" call
7344           creates it (typically because MODE includes the "O_CREAT" flag),
7345           then the value of PERMS specifies the permissions of the newly
7346           created file.  If you omit the PERMS argument to "sysopen", Perl
7347           uses the octal value 0666.  These permission values need to be in
7348           octal, and are modified by your process's current "umask".
7349
7350           In many systems the "O_EXCL" flag is available for opening files in
7351           exclusive mode.  This is not locking: exclusiveness means here that
7352           if the file already exists, "sysopen" fails.  "O_EXCL" may not work
7353           on network filesystems, and has no effect unless the "O_CREAT" flag
7354           is set as well.  Setting "O_CREAT|O_EXCL" prevents the file from
7355           being opened if it is a symbolic link.  It does not protect against
7356           symbolic links in the file's path.
7357
7358           Sometimes you may want to truncate an already-existing file.  This
7359           can be done using the "O_TRUNC" flag.  The behavior of "O_TRUNC"
7360           with "O_RDONLY" is undefined.
7361
7362           You should seldom if ever use 0644 as argument to "sysopen",
7363           because that takes away the user's option to have a more permissive
7364           umask.  Better to omit it.  See "umask" for more on this.
7365
7366           This function has no direct relation to the usage of "sysread",
7367           "syswrite", or "sysseek".  A handle opened with this function can
7368           be used with buffered IO just as one opened with "open" can be used
7369           with unbuffered IO.
7370
7371           Note that under Perls older than 5.8.0, "sysopen" depends on the
7372           fdopen(3) C library function.  On many Unix systems, fdopen(3) is
7373           known to fail when file descriptors exceed a certain value,
7374           typically 255.  If you need more file descriptors than that,
7375           consider using the "POSIX::open" function.  For Perls 5.8.0 and
7376           later, PerlIO is (most often) the default.
7377
7378           See perlopentut for a kinder, gentler explanation of opening files.
7379
7380           Portability issues: "sysopen" in perlport.
7381
7382       sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7383       sysread FILEHANDLE,SCALAR,LENGTH
7384           Attempts to read LENGTH bytes of data into variable SCALAR from the
7385           specified FILEHANDLE, using read(2).  It bypasses any PerlIO layers
7386           including buffered IO (but is affected by the presence of the
7387           ":utf8" layer as described later), so mixing this with other kinds
7388           of reads, "print", "write", "seek", "tell", or "eof" can cause
7389           confusion because the ":perlio" or ":crlf" layers usually buffer
7390           data.  Returns the number of bytes actually read, 0 at end of file,
7391           or undef if there was an error (in the latter case $! is also set).
7392           SCALAR will be grown or shrunk so that the last byte actually read
7393           is the last byte of the scalar after the read.
7394
7395           An OFFSET may be specified to place the read data at some place in
7396           the string other than the beginning.  A negative OFFSET specifies
7397           placement at that many characters counting backwards from the end
7398           of the string.  A positive OFFSET greater than the length of SCALAR
7399           results in the string being padded to the required size with "\0"
7400           bytes before the result of the read is appended.
7401
7402           There is no syseof() function, which is ok, since "eof" doesn't
7403           work well on device files (like ttys) anyway.  Use "sysread" and
7404           check for a return value of 0 to decide whether you're done.
7405
7406           Note that if the filehandle has been marked as ":utf8", "sysread"
7407           will throw an exception.  The ":encoding(...)" layer implicitly
7408           introduces the ":utf8" layer.  See "binmode", "open", and the open
7409           pragma.
7410
7411       sysseek FILEHANDLE,POSITION,WHENCE
7412           Sets FILEHANDLE's system position in bytes using lseek(2).
7413           FILEHANDLE may be an expression whose value gives the name of the
7414           filehandle.  The values for WHENCE are 0 to set the new position to
7415           POSITION; 1 to set it to the current position plus POSITION; and 2
7416           to set it to EOF plus POSITION, typically negative.
7417
7418           Note the emphasis on bytes: even if the filehandle has been set to
7419           operate on characters (for example using the ":encoding(UTF-8)" I/O
7420           layer), the "seek", "tell", and "sysseek" family of functions use
7421           byte offsets, not character offsets, because seeking to a character
7422           offset would be very slow in a UTF-8 file.
7423
7424           "sysseek" bypasses normal buffered IO, so mixing it with reads
7425           other than "sysread" (for example "readline" or "read"), "print",
7426           "write", "seek", "tell", or "eof" may cause confusion.
7427
7428           For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR",
7429           and "SEEK_END" (start of the file, current position, end of the
7430           file) from the Fcntl module.  Use of the constants is also more
7431           portable than relying on 0, 1, and 2.  For example to define a
7432           "systell" function:
7433
7434               use Fcntl 'SEEK_CUR';
7435               sub systell { sysseek($_[0], 0, SEEK_CUR) }
7436
7437           Returns the new position, or the undefined value on failure.  A
7438           position of zero is returned as the string "0 but true"; thus
7439           "sysseek" returns true on success and false on failure, yet you can
7440           still easily determine the new position.
7441
7442       system LIST
7443       system PROGRAM LIST
7444           Does exactly the same thing as "exec", except that a fork is done
7445           first and the parent process waits for the child process to exit.
7446           Note that argument processing varies depending on the number of
7447           arguments.  If there is more than one argument in LIST, or if LIST
7448           is an array with more than one value, starts the program given by
7449           the first element of the list with arguments given by the rest of
7450           the list.  If there is only one scalar argument, the argument is
7451           checked for shell metacharacters, and if there are any, the entire
7452           argument is passed to the system's command shell for parsing (this
7453           is "/bin/sh -c" on Unix platforms, but varies on other platforms).
7454           If there are no shell metacharacters in the argument, it is split
7455           into words and passed directly to "execvp", which is more
7456           efficient.  On Windows, only the "system PROGRAM LIST" syntax will
7457           reliably avoid using the shell; "system LIST", even with more than
7458           one element, will fall back to the shell if the first spawn fails.
7459
7460           Perl will attempt to flush all files opened for output before any
7461           operation that may do a fork, but this may not be supported on some
7462           platforms (see perlport).  To be safe, you may need to set $|
7463           ($AUTOFLUSH in English) or call the "autoflush" method of
7464           "IO::Handle" on any open handles.
7465
7466           The return value is the exit status of the program as returned by
7467           the "wait" call.  To get the actual exit value, shift right by
7468           eight (see below).  See also "exec".  This is not what you want to
7469           use to capture the output from a command; for that you should use
7470           merely backticks or "qx//", as described in "`STRING`" in perlop.
7471           Return value of -1 indicates a failure to start the program or an
7472           error of the wait(2) system call (inspect $! for the reason).
7473
7474           If you'd like to make "system" (and many other bits of Perl) die on
7475           error, have a look at the autodie pragma.
7476
7477           Like "exec", "system" allows you to lie to a program about its name
7478           if you use the "system PROGRAM LIST" syntax.  Again, see "exec".
7479
7480           Since "SIGINT" and "SIGQUIT" are ignored during the execution of
7481           "system", if you expect your program to terminate on receipt of
7482           these signals you will need to arrange to do so yourself based on
7483           the return value.
7484
7485               my @args = ("command", "arg1", "arg2");
7486               system(@args) == 0
7487                   or die "system @args failed: $?";
7488
7489           If you'd like to manually inspect "system"'s failure, you can check
7490           all possible failure modes by inspecting $? like this:
7491
7492               if ($? == -1) {
7493                   print "failed to execute: $!\n";
7494               }
7495               elsif ($? & 127) {
7496                   printf "child died with signal %d, %s coredump\n",
7497                       ($? & 127),  ($? & 128) ? 'with' : 'without';
7498               }
7499               else {
7500                   printf "child exited with value %d\n", $? >> 8;
7501               }
7502
7503           Alternatively, you may inspect the value of
7504           "${^CHILD_ERROR_NATIVE}" with the "W*()" calls from the POSIX
7505           module.
7506
7507           When "system"'s arguments are executed indirectly by the shell,
7508           results and return codes are subject to its quirks.  See "`STRING`"
7509           in perlop and "exec" for details.
7510
7511           Since "system" does a "fork" and "wait" it may affect a "SIGCHLD"
7512           handler.  See perlipc for details.
7513
7514           Portability issues: "system" in perlport.
7515
7516       syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
7517       syswrite FILEHANDLE,SCALAR,LENGTH
7518       syswrite FILEHANDLE,SCALAR
7519           Attempts to write LENGTH bytes of data from variable SCALAR to the
7520           specified FILEHANDLE, using write(2).  If LENGTH is not specified,
7521           writes whole SCALAR.  It bypasses any PerlIO layers including
7522           buffered IO (but is affected by the presence of the ":utf8" layer
7523           as described later), so mixing this with reads (other than
7524           "sysread)"), "print", "write", "seek", "tell", or "eof" may cause
7525           confusion because the ":perlio" and ":crlf" layers usually buffer
7526           data.  Returns the number of bytes actually written, or "undef" if
7527           there was an error (in this case the errno variable $! is also
7528           set).  If the LENGTH is greater than the data available in the
7529           SCALAR after the OFFSET, only as much data as is available will be
7530           written.
7531
7532           An OFFSET may be specified to write the data from some part of the
7533           string other than the beginning.  A negative OFFSET specifies
7534           writing that many characters counting backwards from the end of the
7535           string.  If SCALAR is of length zero, you can only use an OFFSET of
7536           0.
7537
7538           WARNING: If the filehandle is marked ":utf8", "syswrite" will raise
7539           an exception.  The ":encoding(...)" layer implicitly introduces the
7540           ":utf8" layer.  Alternately, if the handle is not marked with an
7541           encoding but you attempt to write characters with code points over
7542           255, raises an exception.  See "binmode", "open", and the open
7543           pragma.
7544
7545       tell FILEHANDLE
7546       tell
7547           Returns the current position in bytes for FILEHANDLE, or -1 on
7548           error.  FILEHANDLE may be an expression whose value gives the name
7549           of the actual filehandle.  If FILEHANDLE is omitted, assumes the
7550           file last read.
7551
7552           Note the emphasis on bytes: even if the filehandle has been set to
7553           operate on characters (for example using the ":encoding(UTF-8)" I/O
7554           layer), the "seek", "tell", and "sysseek" family of functions use
7555           byte offsets, not character offsets, because seeking to a character
7556           offset would be very slow in a UTF-8 file.
7557
7558           The return value of "tell" for the standard streams like the STDIN
7559           depends on the operating system: it may return -1 or something
7560           else.  "tell" on pipes, fifos, and sockets usually returns -1.
7561
7562           There is no "systell" function.  Use "sysseek($fh, 0, 1)" for that.
7563
7564           Do not use "tell" (or other buffered I/O operations) on a
7565           filehandle that has been manipulated by "sysread", "syswrite", or
7566           "sysseek".  Those functions ignore the buffering, while "tell" does
7567           not.
7568
7569       telldir DIRHANDLE
7570           Returns the current position of the "readdir" routines on
7571           DIRHANDLE.  Value may be given to "seekdir" to access a particular
7572           location in a directory.  "telldir" has the same caveats about
7573           possible directory compaction as the corresponding system library
7574           routine.
7575
7576       tie VARIABLE,CLASSNAME,LIST
7577           This function binds a variable to a package class that will provide
7578           the implementation for the variable.  VARIABLE is the name of the
7579           variable to be enchanted.  CLASSNAME is the name of a class
7580           implementing objects of correct type.  Any additional arguments are
7581           passed to the appropriate constructor method of the class (meaning
7582           "TIESCALAR", "TIEHANDLE", "TIEARRAY", or "TIEHASH").  Typically
7583           these are arguments such as might be passed to the dbm_open(3)
7584           function of C.  The object returned by the constructor is also
7585           returned by the "tie" function, which would be useful if you want
7586           to access other methods in CLASSNAME.
7587
7588           Note that functions such as "keys" and "values" may return huge
7589           lists when used on large objects, like DBM files.  You may prefer
7590           to use the "each" function to iterate over such.  Example:
7591
7592               # print out history file offsets
7593               use NDBM_File;
7594               tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
7595               while (my ($key,$val) = each %HIST) {
7596                   print $key, ' = ', unpack('L', $val), "\n";
7597               }
7598
7599           A class implementing a hash should have the following methods:
7600
7601               TIEHASH classname, LIST
7602               FETCH this, key
7603               STORE this, key, value
7604               DELETE this, key
7605               CLEAR this
7606               EXISTS this, key
7607               FIRSTKEY this
7608               NEXTKEY this, lastkey
7609               SCALAR this
7610               DESTROY this
7611               UNTIE this
7612
7613           A class implementing an ordinary array should have the following
7614           methods:
7615
7616               TIEARRAY classname, LIST
7617               FETCH this, key
7618               STORE this, key, value
7619               FETCHSIZE this
7620               STORESIZE this, count
7621               CLEAR this
7622               PUSH this, LIST
7623               POP this
7624               SHIFT this
7625               UNSHIFT this, LIST
7626               SPLICE this, offset, length, LIST
7627               EXTEND this, count
7628               DELETE this, key
7629               EXISTS this, key
7630               DESTROY this
7631               UNTIE this
7632
7633           A class implementing a filehandle should have the following
7634           methods:
7635
7636               TIEHANDLE classname, LIST
7637               READ this, scalar, length, offset
7638               READLINE this
7639               GETC this
7640               WRITE this, scalar, length, offset
7641               PRINT this, LIST
7642               PRINTF this, format, LIST
7643               BINMODE this
7644               EOF this
7645               FILENO this
7646               SEEK this, position, whence
7647               TELL this
7648               OPEN this, mode, LIST
7649               CLOSE this
7650               DESTROY this
7651               UNTIE this
7652
7653           A class implementing a scalar should have the following methods:
7654
7655               TIESCALAR classname, LIST
7656               FETCH this,
7657               STORE this, value
7658               DESTROY this
7659               UNTIE this
7660
7661           Not all methods indicated above need be implemented.  See perltie,
7662           Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.
7663
7664           Unlike "dbmopen", the "tie" function will not "use" or "require" a
7665           module for you; you need to do that explicitly yourself.  See
7666           DB_File or the Config module for interesting "tie" implementations.
7667
7668           For further details see perltie, "tied".
7669
7670       tied VARIABLE
7671           Returns a reference to the object underlying VARIABLE (the same
7672           value that was originally returned by the "tie" call that bound the
7673           variable to a package.)  Returns the undefined value if VARIABLE
7674           isn't tied to a package.
7675
7676       time
7677           Returns the number of non-leap seconds since whatever time the
7678           system considers to be the epoch, suitable for feeding to "gmtime"
7679           and "localtime".  On most systems the epoch is 00:00:00 UTC,
7680           January 1, 1970; a prominent exception being Mac OS Classic which
7681           uses 00:00:00, January 1, 1904 in the current local time zone for
7682           its epoch.
7683
7684           For measuring time in better granularity than one second, use the
7685           Time::HiRes module from Perl 5.8 onwards (or from CPAN before
7686           then), or, if you have gettimeofday(2), you may be able to use the
7687           "syscall" interface of Perl.  See perlfaq8 for details.
7688
7689           For date and time processing look at the many related modules on
7690           CPAN.  For a comprehensive date and time representation look at the
7691           DateTime module.
7692
7693       times
7694           Returns a four-element list giving the user and system times in
7695           seconds for this process and any exited children of this process.
7696
7697               my ($user,$system,$cuser,$csystem) = times;
7698
7699           In scalar context, "times" returns $user.
7700
7701           Children's times are only included for terminated children.
7702
7703           Portability issues: "times" in perlport.
7704
7705       tr///
7706           The transliteration operator.  Same as "y///".  See "Quote-Like
7707           Operators" in perlop.
7708
7709       truncate FILEHANDLE,LENGTH
7710       truncate EXPR,LENGTH
7711           Truncates the file opened on FILEHANDLE, or named by EXPR, to the
7712           specified length.  Raises an exception if truncate isn't
7713           implemented on your system.  Returns true if successful, "undef" on
7714           error.
7715
7716           The behavior is undefined if LENGTH is greater than the length of
7717           the file.
7718
7719           The position in the file of FILEHANDLE is left unchanged.  You may
7720           want to call seek before writing to the file.
7721
7722           Portability issues: "truncate" in perlport.
7723
7724       uc EXPR
7725       uc  Returns an uppercased version of EXPR.  This is the internal
7726           function implementing the "\U" escape in double-quoted strings.  It
7727           does not attempt to do titlecase mapping on initial letters.  See
7728           "ucfirst" for that.
7729
7730           If EXPR is omitted, uses $_.
7731
7732           This function behaves the same way under various pragmas, such as
7733           in a locale, as "lc" does.
7734
7735       ucfirst EXPR
7736       ucfirst
7737           Returns the value of EXPR with the first character in uppercase
7738           (titlecase in Unicode).  This is the internal function implementing
7739           the "\u" escape in double-quoted strings.
7740
7741           If EXPR is omitted, uses $_.
7742
7743           This function behaves the same way under various pragmas, such as
7744           in a locale, as "lc" does.
7745
7746       umask EXPR
7747       umask
7748           Sets the umask for the process to EXPR and returns the previous
7749           value.  If EXPR is omitted, merely returns the current umask.
7750
7751           The Unix permission "rwxr-x---" is represented as three sets of
7752           three bits, or three octal digits: 0750 (the leading 0 indicates
7753           octal and isn't one of the digits).  The "umask" value is such a
7754           number representing disabled permissions bits.  The permission (or
7755           "mode") values you pass "mkdir" or "sysopen" are modified by your
7756           umask, so even if you tell "sysopen" to create a file with
7757           permissions 0777, if your umask is 0022, then the file will
7758           actually be created with permissions 0755.  If your "umask" were
7759           0027 (group can't write; others can't read, write, or execute),
7760           then passing "sysopen" 0666 would create a file with mode 0640
7761           (because "0666 &~ 027" is 0640).
7762
7763           Here's some advice: supply a creation mode of 0666 for regular
7764           files (in "sysopen") and one of 0777 for directories (in "mkdir")
7765           and executable files.  This gives users the freedom of choice: if
7766           they want protected files, they might choose process umasks of 022,
7767           027, or even the particularly antisocial mask of 077.  Programs
7768           should rarely if ever make policy decisions better left to the
7769           user.  The exception to this is when writing files that should be
7770           kept private: mail files, web browser cookies, .rhosts files, and
7771           so on.
7772
7773           If umask(2) is not implemented on your system and you are trying to
7774           restrict access for yourself (i.e., "(EXPR & 0700) > 0"), raises an
7775           exception.  If umask(2) is not implemented and you are not trying
7776           to restrict access for yourself, returns "undef".
7777
7778           Remember that a umask is a number, usually given in octal; it is
7779           not a string of octal digits.  See also "oct", if all you have is a
7780           string.
7781
7782           Portability issues: "umask" in perlport.
7783
7784       undef EXPR
7785       undef
7786           Undefines the value of EXPR, which must be an lvalue.  Use only on
7787           a scalar value, an array (using "@"), a hash (using "%"), a
7788           subroutine (using "&"), or a typeglob (using "*").  Saying "undef
7789           $hash{$key}" will probably not do what you expect on most
7790           predefined variables or DBM list values, so don't do that; see
7791           "delete".  Always returns the undefined value.  You can omit the
7792           EXPR, in which case nothing is undefined, but you still get an
7793           undefined value that you could, for instance, return from a
7794           subroutine, assign to a variable, or pass as a parameter.
7795           Examples:
7796
7797               undef $foo;
7798               undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
7799               undef @ary;
7800               undef %hash;
7801               undef &mysub;
7802               undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
7803               return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
7804               select undef, undef, undef, 0.25;
7805               my ($x, $y, undef, $z) = foo();    # Ignore third value returned
7806
7807           Note that this is a unary operator, not a list operator.
7808
7809       unlink LIST
7810       unlink
7811           Deletes a list of files.  On success, it returns the number of
7812           files it successfully deleted.  On failure, it returns false and
7813           sets $! (errno):
7814
7815               my $unlinked = unlink 'a', 'b', 'c';
7816               unlink @goners;
7817               unlink glob "*.bak";
7818
7819           On error, "unlink" will not tell you which files it could not
7820           remove.  If you want to know which files you could not remove, try
7821           them one at a time:
7822
7823                foreach my $file ( @goners ) {
7824                    unlink $file or warn "Could not unlink $file: $!";
7825                }
7826
7827           Note: "unlink" will not attempt to delete directories unless you
7828           are superuser and the -U flag is supplied to Perl.  Even if these
7829           conditions are met, be warned that unlinking a directory can
7830           inflict damage on your filesystem.  Finally, using "unlink" on
7831           directories is not supported on many operating systems.  Use
7832           "rmdir" instead.
7833
7834           If LIST is omitted, "unlink" uses $_.
7835
7836       unpack TEMPLATE,EXPR
7837       unpack TEMPLATE
7838           "unpack" does the reverse of "pack": it takes a string and expands
7839           it out into a list of values.  (In scalar context, it returns
7840           merely the first value produced.)
7841
7842           If EXPR is omitted, unpacks the $_ string.  See perlpacktut for an
7843           introduction to this function.
7844
7845           The string is broken into chunks described by the TEMPLATE.  Each
7846           chunk is converted separately to a value.  Typically, either the
7847           string is a result of "pack", or the characters of the string
7848           represent a C structure of some kind.
7849
7850           The TEMPLATE has the same format as in the "pack" function.  Here's
7851           a subroutine that does substring:
7852
7853               sub substr {
7854                   my ($what, $where, $howmuch) = @_;
7855                   unpack("x$where a$howmuch", $what);
7856               }
7857
7858           and then there's
7859
7860               sub ordinal { unpack("W",$_[0]); } # same as ord()
7861
7862           In addition to fields allowed in "pack", you may prefix a field
7863           with a %<number> to indicate that you want a <number>-bit checksum
7864           of the items instead of the items themselves.  Default is a 16-bit
7865           checksum.  The checksum is calculated by summing numeric values of
7866           expanded values (for string fields the sum of "ord($char)" is
7867           taken; for bit fields the sum of zeroes and ones).
7868
7869           For example, the following computes the same number as the System V
7870           sum program:
7871
7872               my $checksum = do {
7873                   local $/;  # slurp!
7874                   unpack("%32W*", readline) % 65535;
7875               };
7876
7877           The following efficiently counts the number of set bits in a bit
7878           vector:
7879
7880               my $setbits = unpack("%32b*", $selectmask);
7881
7882           The "p" and "P" formats should be used with care.  Since Perl has
7883           no way of checking whether the value passed to "unpack" corresponds
7884           to a valid memory location, passing a pointer value that's not
7885           known to be valid is likely to have disastrous consequences.
7886
7887           If there are more pack codes or if the repeat count of a field or a
7888           group is larger than what the remainder of the input string allows,
7889           the result is not well defined: the repeat count may be decreased,
7890           or "unpack" may produce empty strings or zeros, or it may raise an
7891           exception.  If the input string is longer than one described by the
7892           TEMPLATE, the remainder of that input string is ignored.
7893
7894           See "pack" for more examples and notes.
7895
7896       unshift ARRAY,LIST
7897           Does the opposite of a "shift".  Or the opposite of a "push",
7898           depending on how you look at it.  Prepends list to the front of the
7899           array and returns the new number of elements in the array.
7900
7901               unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7902
7903           Note the LIST is prepended whole, not one element at a time, so the
7904           prepended elements stay in the same order.  Use "reverse" to do the
7905           reverse.
7906
7907           Starting with Perl 5.14, an experimental feature allowed "unshift"
7908           to take a scalar expression. This experiment has been deemed
7909           unsuccessful, and was removed as of Perl 5.24.
7910
7911       untie VARIABLE
7912           Breaks the binding between a variable and a package.  (See tie.)
7913           Has no effect if the variable is not tied.
7914
7915       use Module VERSION LIST
7916       use Module VERSION
7917       use Module LIST
7918       use Module
7919       use VERSION
7920           Imports some semantics into the current package from the named
7921           module, generally by aliasing certain subroutine or variable names
7922           into your package.  It is exactly equivalent to
7923
7924               BEGIN { require Module; Module->import( LIST ); }
7925
7926           except that Module must be a bareword.  The importation can be made
7927           conditional by using the if module.
7928
7929           In the "use VERSION" form, VERSION may be either a v-string such as
7930           v5.24.1, which will be compared to $^V (aka $PERL_VERSION), or a
7931           numeric argument of the form 5.024001, which will be compared to
7932           $].  An exception is raised if VERSION is greater than the version
7933           of the current Perl interpreter; Perl will not attempt to parse the
7934           rest of the file.  Compare with "require", which can do a similar
7935           check at run time.  Symmetrically, "no VERSION" allows you to
7936           specify that you want a version of Perl older than the specified
7937           one.
7938
7939           Specifying VERSION as a numeric argument of the form 5.024001
7940           should generally be avoided as older less readable syntax compared
7941           to v5.24.1. Before perl 5.8.0 released in 2002 the more verbose
7942           numeric form was the only supported syntax, which is why you might
7943           see it in
7944
7945               use v5.24.1;    # compile time version check
7946               use 5.24.1;     # ditto
7947               use 5.024_001;  # ditto; older syntax compatible with perl 5.6
7948
7949           This is often useful if you need to check the current Perl version
7950           before "use"ing library modules that won't work with older versions
7951           of Perl.  (We try not to do this more than we have to.)
7952
7953           "use VERSION" also lexically enables all features available in the
7954           requested version as defined by the feature pragma, disabling any
7955           features not in the requested version's feature bundle.  See
7956           feature.  Similarly, if the specified Perl version is greater than
7957           or equal to 5.12.0, strictures are enabled lexically as with "use
7958           strict".  Any explicit use of "use strict" or "no strict" overrides
7959           "use VERSION", even if it comes before it.  Later use of "use
7960           VERSION" will override all behavior of a previous "use VERSION",
7961           possibly removing the "strict" and "feature" added by "use
7962           VERSION".  "use VERSION" does not load the feature.pm or strict.pm
7963           files.
7964
7965           The "BEGIN" forces the "require" and "import" to happen at compile
7966           time.  The "require" makes sure the module is loaded into memory if
7967           it hasn't been yet.  The "import" is not a builtin; it's just an
7968           ordinary static method call into the "Module" package to tell the
7969           module to import the list of features back into the current
7970           package.  The module can implement its "import" method any way it
7971           likes, though most modules just choose to derive their "import"
7972           method via inheritance from the "Exporter" class that is defined in
7973           the "Exporter" module.  See Exporter.  If no "import" method can be
7974           found, then the call is skipped, even if there is an AUTOLOAD
7975           method.
7976
7977           If you do not want to call the package's "import" method (for
7978           instance, to stop your namespace from being altered), explicitly
7979           supply the empty list:
7980
7981               use Module ();
7982
7983           That is exactly equivalent to
7984
7985               BEGIN { require Module }
7986
7987           If the VERSION argument is present between Module and LIST, then
7988           the "use" will call the "VERSION" method in class Module with the
7989           given version as an argument:
7990
7991               use Module 12.34;
7992
7993           is equivalent to:
7994
7995               BEGIN { require Module; Module->VERSION(12.34) }
7996
7997           The default "VERSION" method, inherited from the "UNIVERSAL" class,
7998           croaks if the given version is larger than the value of the
7999           variable $Module::VERSION.
8000
8001           The VERSION argument cannot be an arbitrary expression.  It only
8002           counts as a VERSION argument if it is a version number literal,
8003           starting with either a digit or "v" followed by a digit.  Anything
8004           that doesn't look like a version literal will be parsed as the
8005           start of the LIST.  Nevertheless, many attempts to use an arbitrary
8006           expression as a VERSION argument will appear to work, because
8007           Exporter's "import" method handles numeric arguments specially,
8008           performing version checks rather than treating them as things to
8009           export.
8010
8011           Again, there is a distinction between omitting LIST ("import"
8012           called with no arguments) and an explicit empty LIST "()" ("import"
8013           not called).  Note that there is no comma after VERSION!
8014
8015           Because this is a wide-open interface, pragmas (compiler
8016           directives) are also implemented this way.  Some of the currently
8017           implemented pragmas are:
8018
8019               use constant;
8020               use diagnostics;
8021               use integer;
8022               use sigtrap  qw(SEGV BUS);
8023               use strict   qw(subs vars refs);
8024               use subs     qw(afunc blurfl);
8025               use warnings qw(all);
8026               use sort     qw(stable);
8027
8028           Some of these pseudo-modules import semantics into the current
8029           block scope (like "strict" or "integer", unlike ordinary modules,
8030           which import symbols into the current package (which are effective
8031           through the end of the file).
8032
8033           Because "use" takes effect at compile time, it doesn't respect the
8034           ordinary flow control of the code being compiled.  In particular,
8035           putting a "use" inside the false branch of a conditional doesn't
8036           prevent it from being processed.  If a module or pragma only needs
8037           to be loaded conditionally, this can be done using the if pragma:
8038
8039               use if $] < 5.008, "utf8";
8040               use if WANT_WARNINGS, warnings => qw(all);
8041
8042           There's a corresponding "no" declaration that unimports meanings
8043           imported by "use", i.e., it calls "Module->unimport(LIST)" instead
8044           of "import".  It behaves just as "import" does with VERSION, an
8045           omitted or empty LIST, or no unimport method being found.
8046
8047               no integer;
8048               no strict 'refs';
8049               no warnings;
8050
8051           Care should be taken when using the "no VERSION" form of "no".  It
8052           is only meant to be used to assert that the running Perl is of a
8053           earlier version than its argument and not to undo the feature-
8054           enabling side effects of "use VERSION".
8055
8056           See perlmodlib for a list of standard modules and pragmas.  See
8057           perlrun for the "-M" and "-m" command-line options to Perl that
8058           give "use" functionality from the command-line.
8059
8060       utime LIST
8061           Changes the access and modification times on each file of a list of
8062           files.  The first two elements of the list must be the NUMERIC
8063           access and modification times, in that order.  Returns the number
8064           of files successfully changed.  The inode change time of each file
8065           is set to the current time.  For example, this code has the same
8066           effect as the Unix touch(1) command when the files already exist
8067           and belong to the user running the program:
8068
8069               #!/usr/bin/perl
8070               my $atime = my $mtime = time;
8071               utime $atime, $mtime, @ARGV;
8072
8073           Since Perl 5.8.0, if the first two elements of the list are
8074           "undef", the utime(2) syscall from your C library is called with a
8075           null second argument.  On most systems, this will set the file's
8076           access and modification times to the current time (i.e., equivalent
8077           to the example above) and will work even on files you don't own
8078           provided you have write permission:
8079
8080               for my $file (@ARGV) {
8081                   utime(undef, undef, $file)
8082                       || warn "Couldn't touch $file: $!";
8083               }
8084
8085           Under NFS this will use the time of the NFS server, not the time of
8086           the local machine.  If there is a time synchronization problem, the
8087           NFS server and local machine will have different times.  The Unix
8088           touch(1) command will in fact normally use this form instead of the
8089           one shown in the first example.
8090
8091           Passing only one of the first two elements as "undef" is equivalent
8092           to passing a 0 and will not have the effect described when both are
8093           "undef".  This also triggers an uninitialized warning.
8094
8095           On systems that support futimes(2), you may pass filehandles among
8096           the files.  On systems that don't support futimes(2), passing
8097           filehandles raises an exception.  Filehandles must be passed as
8098           globs or glob references to be recognized; barewords are considered
8099           filenames.
8100
8101           Portability issues: "utime" in perlport.
8102
8103       values HASH
8104       values ARRAY
8105           In list context, returns a list consisting of all the values of the
8106           named hash.  In Perl 5.12 or later only, will also return a list of
8107           the values of an array; prior to that release, attempting to use an
8108           array argument will produce a syntax error.  In scalar context,
8109           returns the number of values.
8110
8111           Hash entries are returned in an apparently random order.  The
8112           actual random order is specific to a given hash; the exact same
8113           series of operations on two hashes may result in a different order
8114           for each hash.  Any insertion into the hash may change the order,
8115           as will any deletion, with the exception that the most recent key
8116           returned by "each" or "keys" may be deleted without changing the
8117           order.  So long as a given hash is unmodified you may rely on
8118           "keys", "values" and "each" to repeatedly return the same order as
8119           each other.  See "Algorithmic Complexity Attacks" in perlsec for
8120           details on why hash order is randomized.  Aside from the guarantees
8121           provided here the exact details of Perl's hash algorithm and the
8122           hash traversal order are subject to change in any release of Perl.
8123           Tied hashes may behave differently to Perl's hashes with respect to
8124           changes in order on insertion and deletion of items.
8125
8126           As a side effect, calling "values" resets the HASH or ARRAY's
8127           internal iterator (see "each") before yielding the values.  In
8128           particular, calling "values" in void context resets the iterator
8129           with no other overhead.
8130
8131           Apart from resetting the iterator, "values @array" in list context
8132           is the same as plain @array.  (We recommend that you use void
8133           context "keys @array" for this, but reasoned that taking "values
8134           @array" out would require more documentation than leaving it in.)
8135
8136           Note that the values are not copied, which means modifying them
8137           will modify the contents of the hash:
8138
8139               for (values %hash)      { s/foo/bar/g }  # modifies %hash values
8140               for (@hash{keys %hash}) { s/foo/bar/g }  # same
8141
8142           Starting with Perl 5.14, an experimental feature allowed "values"
8143           to take a scalar expression. This experiment has been deemed
8144           unsuccessful, and was removed as of Perl 5.24.
8145
8146           To avoid confusing would-be users of your code who are running
8147           earlier versions of Perl with mysterious syntax errors, put this
8148           sort of thing at the top of your file to signal that your code will
8149           work only on Perls of a recent vintage:
8150
8151               use 5.012;  # so keys/values/each work on arrays
8152
8153           See also "keys", "each", and "sort".
8154
8155       vec EXPR,OFFSET,BITS
8156           Treats the string in EXPR as a bit vector made up of elements of
8157           width BITS and returns the value of the element specified by OFFSET
8158           as an unsigned integer.  BITS therefore specifies the number of
8159           bits that are reserved for each element in the bit vector.  This
8160           must be a power of two from 1 to 32 (or 64, if your platform
8161           supports that).
8162
8163           If BITS is 8, "elements" coincide with bytes of the input string.
8164
8165           If BITS is 16 or more, bytes of the input string are grouped into
8166           chunks of size BITS/8, and each group is converted to a number as
8167           with "pack"/"unpack" with big-endian formats "n"/"N" (and
8168           analogously for BITS==64).  See "pack" for details.
8169
8170           If bits is 4 or less, the string is broken into bytes, then the
8171           bits of each byte are broken into 8/BITS groups.  Bits of a byte
8172           are numbered in a little-endian-ish way, as in 0x01, 0x02, 0x04,
8173           0x08, 0x10, 0x20, 0x40, 0x80.  For example, breaking the single
8174           input byte "chr(0x36)" into two groups gives a list "(0x6, 0x3)";
8175           breaking it into 4 groups gives "(0x2, 0x1, 0x3, 0x0)".
8176
8177           "vec" may also be assigned to, in which case parentheses are needed
8178           to give the expression the correct precedence as in
8179
8180               vec($image, $max_x * $x + $y, 8) = 3;
8181
8182           If the selected element is outside the string, the value 0 is
8183           returned.  If an element off the end of the string is written to,
8184           Perl will first extend the string with sufficiently many zero
8185           bytes.   It is an error to try to write off the beginning of the
8186           string (i.e., negative OFFSET).
8187
8188           If the string happens to be encoded as UTF-8 internally (and thus
8189           has the UTF8 flag set), "vec" tries to convert it to use a one-
8190           byte-per-character internal representation. However, if the string
8191           contains characters with values of 256 or higher, a fatal error
8192           will occur.
8193
8194           Strings created with "vec" can also be manipulated with the logical
8195           operators "|", "&", "^", and "~".  These operators will assume a
8196           bit vector operation is desired when both operands are strings.
8197           See "Bitwise String Operators" in perlop.
8198
8199           The following code will build up an ASCII string saying
8200           'PerlPerlPerl'.  The comments show the string after each step.
8201           Note that this code works in the same way on big-endian or little-
8202           endian machines.
8203
8204               my $foo = '';
8205               vec($foo,  0, 32) = 0x5065726C; # 'Perl'
8206
8207               # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8208               print vec($foo, 0, 8);  # prints 80 == 0x50 == ord('P')
8209
8210               vec($foo,  2, 16) = 0x5065; # 'PerlPe'
8211               vec($foo,  3, 16) = 0x726C; # 'PerlPerl'
8212               vec($foo,  8,  8) = 0x50;   # 'PerlPerlP'
8213               vec($foo,  9,  8) = 0x65;   # 'PerlPerlPe'
8214               vec($foo, 20,  4) = 2;      # 'PerlPerlPe'   . "\x02"
8215               vec($foo, 21,  4) = 7;      # 'PerlPerlPer'
8216                                              # 'r' is "\x72"
8217               vec($foo, 45,  2) = 3;      # 'PerlPerlPer'  . "\x0c"
8218               vec($foo, 93,  1) = 1;      # 'PerlPerlPer'  . "\x2c"
8219               vec($foo, 94,  1) = 1;      # 'PerlPerlPerl'
8220                                              # 'l' is "\x6c"
8221
8222           To transform a bit vector into a string or list of 0's and 1's, use
8223           these:
8224
8225               my $bits = unpack("b*", $vector);
8226               my @bits = split(//, unpack("b*", $vector));
8227
8228           If you know the exact length in bits, it can be used in place of
8229           the "*".
8230
8231           Here is an example to illustrate how the bits actually fall in
8232           place:
8233
8234             #!/usr/bin/perl -wl
8235
8236             print <<'EOT';
8237                                               0         1         2         3
8238                                unpack("V",$_) 01234567890123456789012345678901
8239             ------------------------------------------------------------------
8240             EOT
8241
8242             for $w (0..3) {
8243                 $width = 2**$w;
8244                 for ($shift=0; $shift < $width; ++$shift) {
8245                     for ($off=0; $off < 32/$width; ++$off) {
8246                         $str = pack("B*", "0"x32);
8247                         $bits = (1<<$shift);
8248                         vec($str, $off, $width) = $bits;
8249                         $res = unpack("b*",$str);
8250                         $val = unpack("V", $str);
8251                         write;
8252                     }
8253                 }
8254             }
8255
8256             format STDOUT =
8257             vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8258             $off, $width, $bits, $val, $res
8259             .
8260             __END__
8261
8262           Regardless of the machine architecture on which it runs, the
8263           example above should print the following table:
8264
8265                                               0         1         2         3
8266                                unpack("V",$_) 01234567890123456789012345678901
8267             ------------------------------------------------------------------
8268             vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
8269             vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
8270             vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
8271             vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
8272             vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
8273             vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
8274             vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
8275             vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
8276             vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
8277             vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
8278             vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
8279             vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
8280             vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
8281             vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
8282             vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
8283             vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
8284             vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
8285             vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
8286             vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
8287             vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
8288             vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
8289             vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
8290             vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
8291             vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
8292             vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
8293             vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
8294             vec($_,26, 1) = 1   ==   67108864 00000000000000000000000000100000
8295             vec($_,27, 1) = 1   ==  134217728 00000000000000000000000000010000
8296             vec($_,28, 1) = 1   ==  268435456 00000000000000000000000000001000
8297             vec($_,29, 1) = 1   ==  536870912 00000000000000000000000000000100
8298             vec($_,30, 1) = 1   == 1073741824 00000000000000000000000000000010
8299             vec($_,31, 1) = 1   == 2147483648 00000000000000000000000000000001
8300             vec($_, 0, 2) = 1   ==          1 10000000000000000000000000000000
8301             vec($_, 1, 2) = 1   ==          4 00100000000000000000000000000000
8302             vec($_, 2, 2) = 1   ==         16 00001000000000000000000000000000
8303             vec($_, 3, 2) = 1   ==         64 00000010000000000000000000000000
8304             vec($_, 4, 2) = 1   ==        256 00000000100000000000000000000000
8305             vec($_, 5, 2) = 1   ==       1024 00000000001000000000000000000000
8306             vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
8307             vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
8308             vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
8309             vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
8310             vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
8311             vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
8312             vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
8313             vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
8314             vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
8315             vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
8316             vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
8317             vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
8318             vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
8319             vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
8320             vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
8321             vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
8322             vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
8323             vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
8324             vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
8325             vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
8326             vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
8327             vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
8328             vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
8329             vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
8330             vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
8331             vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
8332             vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
8333             vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
8334             vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
8335             vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
8336             vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
8337             vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
8338             vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
8339             vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
8340             vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
8341             vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
8342             vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
8343             vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
8344             vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
8345             vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
8346             vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
8347             vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
8348             vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
8349             vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
8350             vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
8351             vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
8352             vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
8353             vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
8354             vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
8355             vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
8356             vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
8357             vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
8358             vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
8359             vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
8360             vec($_, 4, 4) = 8   ==     524288 00000000000000000001000000000000
8361             vec($_, 5, 4) = 8   ==    8388608 00000000000000000000000100000000
8362             vec($_, 6, 4) = 8   ==  134217728 00000000000000000000000000010000
8363             vec($_, 7, 4) = 8   == 2147483648 00000000000000000000000000000001
8364             vec($_, 0, 8) = 1   ==          1 10000000000000000000000000000000
8365             vec($_, 1, 8) = 1   ==        256 00000000100000000000000000000000
8366             vec($_, 2, 8) = 1   ==      65536 00000000000000001000000000000000
8367             vec($_, 3, 8) = 1   ==   16777216 00000000000000000000000010000000
8368             vec($_, 0, 8) = 2   ==          2 01000000000000000000000000000000
8369             vec($_, 1, 8) = 2   ==        512 00000000010000000000000000000000
8370             vec($_, 2, 8) = 2   ==     131072 00000000000000000100000000000000
8371             vec($_, 3, 8) = 2   ==   33554432 00000000000000000000000001000000
8372             vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
8373             vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
8374             vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
8375             vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
8376             vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
8377             vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
8378             vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
8379             vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
8380             vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
8381             vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
8382             vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
8383             vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
8384             vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
8385             vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
8386             vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
8387             vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
8388             vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
8389             vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
8390             vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
8391             vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
8392             vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
8393             vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
8394             vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
8395             vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8396
8397       wait
8398           Behaves like wait(2) on your system: it waits for a child process
8399           to terminate and returns the pid of the deceased process, or "-1"
8400           if there are no child processes.  The status is returned in $? and
8401           "${^CHILD_ERROR_NATIVE}".  Note that a return value of "-1" could
8402           mean that child processes are being automatically reaped, as
8403           described in perlipc.
8404
8405           If you use "wait" in your handler for $SIG{CHLD}, it may
8406           accidentally wait for the child created by "qx" or "system".  See
8407           perlipc for details.
8408
8409           Portability issues: "wait" in perlport.
8410
8411       waitpid PID,FLAGS
8412           Waits for a particular child process to terminate and returns the
8413           pid of the deceased process, or "-1" if there is no such child
8414           process.  A non-blocking wait (with WNOHANG in FLAGS) can return 0
8415           if there are child processes matching PID but none have terminated
8416           yet.  The status is returned in $? and "${^CHILD_ERROR_NATIVE}".
8417
8418           A PID of 0 indicates to wait for any child process whose process
8419           group ID is equal to that of the current process.  A PID of less
8420           than "-1" indicates to wait for any child process whose process
8421           group ID is equal to -PID.  A PID of "-1" indicates to wait for any
8422           child process.
8423
8424           If you say
8425
8426               use POSIX ":sys_wait_h";
8427
8428               my $kid;
8429               do {
8430                   $kid = waitpid(-1, WNOHANG);
8431               } while $kid > 0;
8432
8433           or
8434
8435               1 while waitpid(-1, WNOHANG) > 0;
8436
8437           then you can do a non-blocking wait for all pending zombie
8438           processes (see "WAIT" in POSIX).  Non-blocking wait is available on
8439           machines supporting either the waitpid(2) or wait4(2) syscalls.
8440           However, waiting for a particular pid with FLAGS of 0 is
8441           implemented everywhere.  (Perl emulates the system call by
8442           remembering the status values of processes that have exited but
8443           have not been harvested by the Perl script yet.)
8444
8445           Note that on some systems, a return value of "-1" could mean that
8446           child processes are being automatically reaped.  See perlipc for
8447           details, and for other examples.
8448
8449           Portability issues: "waitpid" in perlport.
8450
8451       wantarray
8452           Returns true if the context of the currently executing subroutine
8453           or "eval" is looking for a list value.  Returns false if the
8454           context is looking for a scalar.  Returns the undefined value if
8455           the context is looking for no value (void context).
8456
8457               return unless defined wantarray; # don't bother doing more
8458               my @a = complex_calculation();
8459               return wantarray ? @a : "@a";
8460
8461           "wantarray"'s result is unspecified in the top level of a file, in
8462           a "BEGIN", "UNITCHECK", "CHECK", "INIT" or "END" block, or in a
8463           "DESTROY" method.
8464
8465           This function should have been named wantlist() instead.
8466
8467       warn LIST
8468           Emits a warning, usually by printing it to "STDERR".  "warn"
8469           interprets its operand LIST in the same way as "die", but is
8470           slightly different in what it defaults to when LIST is empty or
8471           makes an empty string.  If it is empty and $@ already contains an
8472           exception value then that value is used after appending
8473           "\t...caught".  If it is empty and $@ is also empty then the string
8474           "Warning: Something's wrong" is used.
8475
8476           By default, the exception derived from the operand LIST is
8477           stringified and printed to "STDERR".  This behaviour can be altered
8478           by installing a $SIG{__WARN__} handler.  If there is such a handler
8479           then no message is automatically printed; it is the handler's
8480           responsibility to deal with the exception as it sees fit (like, for
8481           instance, converting it into a "die").  Most handlers must
8482           therefore arrange to actually display the warnings that they are
8483           not prepared to deal with, by calling "warn" again in the handler.
8484           Note that this is quite safe and will not produce an endless loop,
8485           since "__WARN__" hooks are not called from inside one.
8486
8487           You will find this behavior is slightly different from that of
8488           $SIG{__DIE__} handlers (which don't suppress the error text, but
8489           can instead call "die" again to change it).
8490
8491           Using a "__WARN__" handler provides a powerful way to silence all
8492           warnings (even the so-called mandatory ones).  An example:
8493
8494               # wipe out *all* compile-time warnings
8495               BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
8496               my $foo = 10;
8497               my $foo = 20;          # no warning about duplicate my $foo,
8498                                      # but hey, you asked for it!
8499               # no compile-time or run-time warnings before here
8500               $DOWARN = 1;
8501
8502               # run-time warnings enabled after here
8503               warn "\$foo is alive and $foo!";     # does show up
8504
8505           See perlvar for details on setting %SIG entries and for more
8506           examples.  See the Carp module for other kinds of warnings using
8507           its "carp" and "cluck" functions.
8508
8509       write FILEHANDLE
8510       write EXPR
8511       write
8512           Writes a formatted record (possibly multi-line) to the specified
8513           FILEHANDLE, using the format associated with that file.  By default
8514           the format for a file is the one having the same name as the
8515           filehandle, but the format for the current output channel (see the
8516           "select" function) may be set explicitly by assigning the name of
8517           the format to the $~ variable.
8518
8519           Top of form processing is handled automatically:  if there is
8520           insufficient room on the current page for the formatted record, the
8521           page is advanced by writing a form feed and a special top-of-page
8522           format is used to format the new page header before the record is
8523           written.  By default, the top-of-page format is the name of the
8524           filehandle with "_TOP" appended, or "top" in the current package if
8525           the former does not exist.  This would be a problem with
8526           autovivified filehandles, but it may be dynamically set to the
8527           format of your choice by assigning the name to the $^ variable
8528           while that filehandle is selected.  The number of lines remaining
8529           on the current page is in variable "$-", which can be set to 0 to
8530           force a new page.
8531
8532           If FILEHANDLE is unspecified, output goes to the current default
8533           output channel, which starts out as STDOUT but may be changed by
8534           the "select" operator.  If the FILEHANDLE is an EXPR, then the
8535           expression is evaluated and the resulting string is used to look up
8536           the name of the FILEHANDLE at run time.  For more on formats, see
8537           perlform.
8538
8539           Note that write is not the opposite of "read".  Unfortunately.
8540
8541       y///
8542           The transliteration operator.  Same as "tr///".  See "Quote-Like
8543           Operators" in perlop.
8544
8545   Non-function Keywords by Cross-reference
8546       perldata
8547
8548       __DATA__
8549       __END__
8550           These keywords are documented in "Special Literals" in perldata.
8551
8552       perlmod
8553
8554       BEGIN
8555       CHECK
8556       END
8557       INIT
8558       UNITCHECK
8559           These compile phase keywords are documented in "BEGIN, UNITCHECK,
8560           CHECK, INIT and END" in perlmod.
8561
8562       perlobj
8563
8564       DESTROY
8565           This method keyword is documented in "Destructors" in perlobj.
8566
8567       perlop
8568
8569       and
8570       cmp
8571       eq
8572       ge
8573       gt
8574       le
8575       lt
8576       ne
8577       not
8578       or
8579       x
8580       xor These operators are documented in perlop.
8581
8582       perlsub
8583
8584       AUTOLOAD
8585           This keyword is documented in "Autoloading" in perlsub.
8586
8587       perlsyn
8588
8589       else
8590       elsif
8591       for
8592       foreach
8593       if
8594       unless
8595       until
8596       while
8597           These flow-control keywords are documented in "Compound Statements"
8598           in perlsyn.
8599
8600       elseif
8601           The "else if" keyword is spelled "elsif" in Perl.  There's no
8602           "elif" or "else if" either.  It does parse "elseif", but only to
8603           warn you about not using it.
8604
8605           See the documentation for flow-control keywords in "Compound
8606           Statements" in perlsyn.
8607
8608       default
8609       given
8610       when
8611           These flow-control keywords related to the experimental switch
8612           feature are documented in "Switch Statements" in perlsyn.
8613
8614
8615
8616perl v5.34.0                      2021-10-18                       PERLFUNC(1)
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