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

6       perlvar - Perl predefined variables
7

DESCRIPTION

9   The Syntax of Variable Names
10       Variable names in Perl can have several formats.  Usually, they must
11       begin with a letter or underscore, in which case they can be
12       arbitrarily long (up to an internal limit of 251 characters) and may
13       contain letters, digits, underscores, or the special sequence "::" or
14       "'".  In this case, the part before the last "::" or "'" is taken to be
15       a package qualifier; see perlmod.  A Unicode letter that is not ASCII
16       is not considered to be a letter unless "use utf8" is in effect, and
17       somewhat more complicated rules apply; see "Identifier parsing" in
18       perldata for details.
19
20       Perl variable names may also be a sequence of digits, a single
21       punctuation character, or the two-character sequence: "^" (caret or
22       CIRCUMFLEX ACCENT) followed by any one of the characters "[][A-Z^_?\]".
23       These names are all reserved for special uses by Perl; for example, the
24       all-digits names are used to hold data captured by backreferences after
25       a regular expression match.
26
27       Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
28       preceded by a caret.  These must all be written in the form "${^Foo}";
29       the braces are not optional.  "${^Foo}" denotes the scalar variable
30       whose name is considered to be a control-"F" followed by two "o"'s.
31       These variables are reserved for future special uses by Perl, except
32       for the ones that begin with "^_" (caret-underscore).  No name that
33       begins with "^_" will acquire a special meaning in any future version
34       of Perl; such names may therefore be used safely in programs.  $^_
35       itself, however, is reserved.
36
37       Perl identifiers that begin with digits or punctuation characters are
38       exempt from the effects of the "package" declaration and are always
39       forced to be in package "main"; they are also exempt from "strict
40       'vars'" errors.  A few other names are also exempt in these ways:
41
42           ENV      STDIN
43           INC      STDOUT
44           ARGV     STDERR
45           ARGVOUT
46           SIG
47
48       In particular, the special "${^_XYZ}" variables are always taken to be
49       in package "main", regardless of any "package" declarations presently
50       in scope.
51

SPECIAL VARIABLES

53       The following names have special meaning to Perl.  Most punctuation
54       names have reasonable mnemonics, or analogs in the shells.
55       Nevertheless, if you wish to use long variable names, you need only
56       say:
57
58           use English;
59
60       at the top of your program.  This aliases all the short names to the
61       long names in the current package.  Some even have medium names,
62       generally borrowed from awk.  For more info, please see English.
63
64       Before you continue, note the sort order for variables.  In general, we
65       first list the variables in case-insensitive, almost-lexigraphical
66       order (ignoring the "{" or "^" preceding words, as in "${^UNICODE}" or
67       $^T), although $_ and @_ move up to the top of the pile.  For variables
68       with the same identifier, we list it in order of scalar, array, hash,
69       and bareword.
70
71   General Variables
72       $ARG
73       $_      The default input and pattern-searching space.  The following
74               pairs are equivalent:
75
76                   while (<>) {...}    # equivalent only in while!
77                   while (defined($_ = <>)) {...}
78
79                   /^Subject:/
80                   $_ =~ /^Subject:/
81
82                   tr/a-z/A-Z/
83                   $_ =~ tr/a-z/A-Z/
84
85                   chomp
86                   chomp($_)
87
88               Here are the places where Perl will assume $_ even if you don't
89               use it:
90
91               •  The following functions use $_ as a default argument:
92
93                  abs, alarm, chomp, chop, chr, chroot, cos, defined, eval,
94                  evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length,
95                  log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta,
96                  readlink, readpipe, ref, require, reverse (in scalar context
97                  only), rmdir, say, sin, split (for its second argument),
98                  sqrt, stat, study, uc, ucfirst, unlink, unpack.
99
100               •  All file tests ("-f", "-d") except for "-t", which defaults
101                  to STDIN.  See "-X" in perlfunc
102
103               •  The pattern matching operations "m//", "s///" and "tr///"
104                  (aka "y///") when used without an "=~" operator.
105
106               •  The default iterator variable in a "foreach" loop if no
107                  other variable is supplied.
108
109               •  The implicit iterator variable in the "grep()" and "map()"
110                  functions.
111
112               •  The implicit variable of "given()".
113
114               •  The default place to put the next value or input record when
115                  a "<FH>", "readline", "readdir" or "each" operation's result
116                  is tested by itself as the sole criterion of a "while" test.
117                  Outside a "while" test, this will not happen.
118
119               $_ is a global variable.
120
121               However, between perl v5.10.0 and v5.24.0, it could be used
122               lexically by writing "my $_".  Making $_ refer to the global $_
123               in the same scope was then possible with "our $_".  This
124               experimental feature was removed and is now a fatal error, but
125               you may encounter it in older code.
126
127               Mnemonic: underline is understood in certain operations.
128
129       @ARG
130       @_      Within a subroutine the array @_ contains the parameters passed
131               to that subroutine.  Inside a subroutine, @_ is the default
132               array for the array operators "pop" and "shift".
133
134               See perlsub.
135
136       $LIST_SEPARATOR
137       $"      When an array or an array slice is interpolated into a double-
138               quoted string or a similar context such as "/.../", its
139               elements are separated by this value.  Default is a space.  For
140               example, this:
141
142                   print "The array is: @array\n";
143
144               is equivalent to this:
145
146                   print "The array is: " . join($", @array) . "\n";
147
148               Mnemonic: works in double-quoted context.
149
150       $PROCESS_ID
151       $PID
152       $$      The process number of the Perl running this script.  Though you
153               can set this variable, doing so is generally discouraged,
154               although it can be invaluable for some testing purposes.  It
155               will be reset automatically across "fork()" calls.
156
157               Note for Linux and Debian GNU/kFreeBSD users: Before Perl
158               v5.16.0 perl would emulate POSIX semantics on Linux systems
159               using LinuxThreads, a partial implementation of POSIX Threads
160               that has since been superseded by the Native POSIX Thread
161               Library (NPTL).
162
163               LinuxThreads is now obsolete on Linux, and caching "getpid()"
164               like this made embedding perl unnecessarily complex (since
165               you'd have to manually update the value of $$), so now $$ and
166               "getppid()" will always return the same values as the
167               underlying C library.
168
169               Debian GNU/kFreeBSD systems also used LinuxThreads up until and
170               including the 6.0 release, but after that moved to FreeBSD
171               thread semantics, which are POSIX-like.
172
173               To see if your system is affected by this discrepancy check if
174               "getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL" returns a false
175               value.  NTPL threads preserve the POSIX semantics.
176
177               Mnemonic: same as shells.
178
179       $PROGRAM_NAME
180       $0      Contains the name of the program being executed.
181
182               On some (but not all) operating systems assigning to $0
183               modifies the argument area that the "ps" program sees.  On some
184               platforms you may have to use special "ps" options or a
185               different "ps" to see the changes.  Modifying the $0 is more
186               useful as a way of indicating the current program state than it
187               is for hiding the program you're running.
188
189               Note that there are platform-specific limitations on the
190               maximum length of $0.  In the most extreme case it may be
191               limited to the space occupied by the original $0.
192
193               In some platforms there may be arbitrary amount of padding, for
194               example space characters, after the modified name as shown by
195               "ps".  In some platforms this padding may extend all the way to
196               the original length of the argument area, no matter what you do
197               (this is the case for example with Linux 2.2).
198
199               Note for BSD users: setting $0 does not completely remove
200               "perl" from the ps(1) output.  For example, setting $0 to
201               "foobar" may result in "perl: foobar (perl)" (whether both the
202               "perl: " prefix and the " (perl)" suffix are shown depends on
203               your exact BSD variant and version).  This is an operating
204               system feature, Perl cannot help it.
205
206               In multithreaded scripts Perl coordinates the threads so that
207               any thread may modify its copy of the $0 and the change becomes
208               visible to ps(1) (assuming the operating system plays along).
209               Note that the view of $0 the other threads have will not change
210               since they have their own copies of it.
211
212               If the program has been given to perl via the switches "-e" or
213               "-E", $0 will contain the string "-e".
214
215               On Linux as of perl v5.14.0 the legacy process name will be set
216               with prctl(2), in addition to altering the POSIX name via
217               "argv[0]" as perl has done since version 4.000.  Now system
218               utilities that read the legacy process name such as ps, top and
219               killall will recognize the name you set when assigning to $0.
220               The string you supply will be cut off at 16 bytes, this is a
221               limitation imposed by Linux.
222
223               Mnemonic: same as sh and ksh.
224
225       $REAL_GROUP_ID
226       $GID
227       $(      The real gid of this process.  If you are on a machine that
228               supports membership in multiple groups simultaneously, gives a
229               space separated list of groups you are in.  The first number is
230               the one returned by "getgid()", and the subsequent ones by
231               "getgroups()", one of which may be the same as the first
232               number.
233
234               However, a value assigned to $( must be a single number used to
235               set the real gid.  So the value given by $( should not be
236               assigned back to $( without being forced numeric, such as by
237               adding zero.  Note that this is different to the effective gid
238               ($)) which does take a list.
239
240               You can change both the real gid and the effective gid at the
241               same time by using "POSIX::setgid()".  Changes to $( require a
242               check to $!  to detect any possible errors after an attempted
243               change.
244
245               Mnemonic: parentheses are used to group things.  The real gid
246               is the group you left, if you're running setgid.
247
248       $EFFECTIVE_GROUP_ID
249       $EGID
250       $)      The effective gid of this process.  If you are on a machine
251               that supports membership in multiple groups simultaneously,
252               gives a space separated list of groups you are in.  The first
253               number is the one returned by "getegid()", and the subsequent
254               ones by "getgroups()", one of which may be the same as the
255               first number.
256
257               Similarly, a value assigned to $) must also be a space-
258               separated list of numbers.  The first number sets the effective
259               gid, and the rest (if any) are passed to "setgroups()".  To get
260               the effect of an empty list for "setgroups()", just repeat the
261               new effective gid; that is, to force an effective gid of 5 and
262               an effectively empty "setgroups()" list, say " $) = "5 5" ".
263
264               You can change both the effective gid and the real gid at the
265               same time by using "POSIX::setgid()" (use only a single numeric
266               argument).  Changes to $) require a check to $! to detect any
267               possible errors after an attempted change.
268
269               $<, $>, $( and $) can be set only on machines that support the
270               corresponding set[re][ug]id() routine.  $( and $) can be
271               swapped only on machines supporting "setregid()".
272
273               Mnemonic: parentheses are used to group things.  The effective
274               gid is the group that's right for you, if you're running
275               setgid.
276
277       $REAL_USER_ID
278       $UID
279       $<      The real uid of this process.  You can change both the real uid
280               and the effective uid at the same time by using
281               "POSIX::setuid()".  Since changes to $< require a system call,
282               check $! after a change attempt to detect any possible errors.
283
284               Mnemonic: it's the uid you came from, if you're running setuid.
285
286       $EFFECTIVE_USER_ID
287       $EUID
288       $>      The effective uid of this process.  For example:
289
290                   $< = $>;            # set real to effective uid
291                   ($<,$>) = ($>,$<);  # swap real and effective uids
292
293               You can change both the effective uid and the real uid at the
294               same time by using "POSIX::setuid()".  Changes to $> require a
295               check to $! to detect any possible errors after an attempted
296               change.
297
298               $< and $> can be swapped only on machines supporting
299               "setreuid()".
300
301               Mnemonic: it's the uid you went to, if you're running setuid.
302
303       $SUBSCRIPT_SEPARATOR
304       $SUBSEP
305       $;      The subscript separator for multidimensional array emulation.
306               If you refer to a hash element as
307
308                   $foo{$x,$y,$z}
309
310               it really means
311
312                   $foo{join($;, $x, $y, $z)}
313
314               But don't put
315
316                   @foo{$x,$y,$z}      # a slice--note the @
317
318               which means
319
320                   ($foo{$x},$foo{$y},$foo{$z})
321
322               Default is "\034", the same as SUBSEP in awk.  If your keys
323               contain binary data there might not be any safe value for $;.
324
325               Consider using "real" multidimensional arrays as described in
326               perllol.
327
328               Mnemonic: comma (the syntactic subscript separator) is a semi-
329               semicolon.
330
331       $a
332       $b      Special package variables when using "sort()", see "sort" in
333               perlfunc.  Because of this specialness $a and $b don't need to
334               be declared (using "use vars", or "our()") even when using the
335               "strict 'vars'" pragma.  Don't lexicalize them with "my $a" or
336               "my $b" if you want to be able to use them in the "sort()"
337               comparison block or function.
338
339       %ENV    The hash %ENV contains your current environment.  Setting a
340               value in "ENV" changes the environment for any child processes
341               you subsequently "fork()" off.
342
343               As of v5.18.0, both keys and values stored in %ENV are
344               stringified.
345
346                   my $foo = 1;
347                   $ENV{'bar'} = \$foo;
348                   if( ref $ENV{'bar'} ) {
349                       say "Pre 5.18.0 Behaviour";
350                   } else {
351                       say "Post 5.18.0 Behaviour";
352                   }
353
354               Previously, only child processes received stringified values:
355
356                   my $foo = 1;
357                   $ENV{'bar'} = \$foo;
358
359                   # Always printed 'non ref'
360                   system($^X, '-e',
361                          q/print ( ref $ENV{'bar'}  ? 'ref' : 'non ref' ) /);
362
363               This happens because you can't really share arbitrary data
364               structures with foreign processes.
365
366       $OLD_PERL_VERSION
367       $]      The revision, version, and subversion of the Perl interpreter,
368               represented as a decimal of the form 5.XXXYYY, where XXX is the
369               version / 1e3 and YYY is the subversion / 1e6.  For example,
370               Perl v5.10.1 would be "5.010001".
371
372               This variable can be used to determine whether the Perl
373               interpreter executing a script is in the right range of
374               versions:
375
376                   warn "No PerlIO!\n" if "$]" < 5.008;
377
378               When comparing $], numeric comparison operators should be used,
379               but the variable should be stringified first to avoid issues
380               where its original numeric value is inaccurate.
381
382               See also the documentation of "use VERSION" and "require
383               VERSION" for a convenient way to fail if the running Perl
384               interpreter is too old.
385
386               See "$^V" for a representation of the Perl version as a version
387               object, which allows more flexible string comparisons.
388
389               The main advantage of $] over $^V is that it works the same on
390               any version of Perl.  The disadvantages are that it can't
391               easily be compared to versions in other formats (e.g. literal
392               v-strings, "v1.2.3" or version objects) and numeric comparisons
393               are subject to the binary floating point representation; it's
394               good for numeric literal version checks and bad for comparing
395               to a variable that hasn't been sanity-checked.
396
397               The $OLD_PERL_VERSION form was added in Perl v5.20.0 for
398               historical reasons but its use is discouraged. (If your reason
399               to use $] is to run code on old perls then referring to it as
400               $OLD_PERL_VERSION would be self-defeating.)
401
402               Mnemonic: Is this version of perl in the right bracket?
403
404       $SYSTEM_FD_MAX
405       $^F     The maximum system file descriptor, ordinarily 2.  System file
406               descriptors are passed to "exec()"ed processes, while higher
407               file descriptors are not.  Also, during an "open()", system
408               file descriptors are preserved even if the "open()" fails
409               (ordinary file descriptors are closed before the "open()" is
410               attempted).  The close-on-exec status of a file descriptor will
411               be decided according to the value of $^F when the corresponding
412               file, pipe, or socket was opened, not the time of the "exec()".
413
414       @F      The array @F contains the fields of each line read in when
415               autosplit mode is turned on.  See perlrun for the -a switch.
416               This array is package-specific, and must be declared or given a
417               full package name if not in package main when running under
418               "strict 'vars'".
419
420       @INC    The array @INC contains the list of places that the "do EXPR",
421               "require", or "use" constructs look for their library files.
422               It initially consists of the arguments to any -I command-line
423               switches, followed by the default Perl library, probably
424               /usr/local/lib/perl.  Prior to Perl 5.26, "." -which represents
425               the current directory, was included in @INC; it has been
426               removed. This change in behavior is documented in
427               "PERL_USE_UNSAFE_INC" and it is not recommended that "." be re-
428               added to @INC.  If you need to modify @INC at runtime, you
429               should use the "use lib" pragma to get the machine-dependent
430               library properly loaded as well:
431
432                   use lib '/mypath/libdir/';
433                   use SomeMod;
434
435               You can also insert hooks into the file inclusion system by
436               putting Perl code directly into @INC.  Those hooks may be
437               subroutine references, array references or blessed objects.
438               See "require" in perlfunc for details.
439
440       %INC    The hash %INC contains entries for each filename included via
441               the "do", "require", or "use" operators.  The key is the
442               filename you specified (with module names converted to
443               pathnames), and the value is the location of the file found.
444               The "require" operator uses this hash to determine whether a
445               particular file has already been included.
446
447               If the file was loaded via a hook (e.g. a subroutine reference,
448               see "require" in perlfunc for a description of these hooks),
449               this hook is by default inserted into %INC in place of a
450               filename.  Note, however, that the hook may have set the %INC
451               entry by itself to provide some more specific info.
452
453       $INPLACE_EDIT
454       $^I     The current value of the inplace-edit extension.  Use "undef"
455               to disable inplace editing.
456
457               Mnemonic: value of -i switch.
458
459       @ISA    Each package contains a special array called @ISA which
460               contains a list of that class's parent classes, if any. This
461               array is simply a list of scalars, each of which is a string
462               that corresponds to a package name. The array is examined when
463               Perl does method resolution, which is covered in perlobj.
464
465               To load packages while adding them to @ISA, see the parent
466               pragma. The discouraged base pragma does this as well, but
467               should not be used except when compatibility with the
468               discouraged fields pragma is required.
469
470       $^M     By default, running out of memory is an untrappable, fatal
471               error.  However, if suitably built, Perl can use the contents
472               of $^M as an emergency memory pool after "die()"ing.  Suppose
473               that your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and
474               used Perl's malloc.  Then
475
476                   $^M = 'a' x (1 << 16);
477
478               would allocate a 64K buffer for use in an emergency.  See the
479               INSTALL file in the Perl distribution for information on how to
480               add custom C compilation flags when compiling perl.  To
481               discourage casual use of this advanced feature, there is no
482               English long name for this variable.
483
484               This variable was added in Perl 5.004.
485
486       $OSNAME
487       $^O     The name of the operating system under which this copy of Perl
488               was built, as determined during the configuration process.  For
489               examples see "PLATFORMS" in perlport.
490
491               The value is identical to $Config{'osname'}.  See also Config
492               and the -V command-line switch documented in perlrun.
493
494               In Windows platforms, $^O is not very helpful: since it is
495               always "MSWin32", it doesn't tell the difference between
496               95/98/ME/NT/2000/XP/CE/.NET.  Use "Win32::GetOSName()" or
497               Win32::GetOSVersion() (see Win32 and perlport) to distinguish
498               between the variants.
499
500               This variable was added in Perl 5.003.
501
502       %SIG    The hash %SIG contains signal handlers for signals.  For
503               example:
504
505                   sub handler {   # 1st argument is signal name
506                       my($sig) = @_;
507                       print "Caught a SIG$sig--shutting down\n";
508                       close(LOG);
509                       exit(0);
510                       }
511
512                   $SIG{'INT'}  = \&handler;
513                   $SIG{'QUIT'} = \&handler;
514                   ...
515                   $SIG{'INT'}  = 'DEFAULT';   # restore default action
516                   $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT
517
518               Using a value of 'IGNORE' usually has the effect of ignoring
519               the signal, except for the "CHLD" signal.  See perlipc for more
520               about this special case.  Using an empty string or "undef" as
521               the value has the same effect as 'DEFAULT'.
522
523               Here are some other examples:
524
525                   $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not
526                                               # recommended)
527                   $SIG{"PIPE"} = \&Plumber;   # just fine; assume current
528                                               # Plumber
529                   $SIG{"PIPE"} = *Plumber;    # somewhat esoteric
530                   $SIG{"PIPE"} = Plumber();   # oops, what did Plumber()
531                                               # return??
532
533               Be sure not to use a bareword as the name of a signal handler,
534               lest you inadvertently call it.
535
536               Using a string that doesn't correspond to any existing function
537               or a glob that doesn't contain a code slot is equivalent to
538               'IGNORE', but a warning is emitted when the handler is being
539               called (the warning is not emitted for the internal hooks
540               described below).
541
542               If your system has the "sigaction()" function then signal
543               handlers are installed using it.  This means you get reliable
544               signal handling.
545
546               The default delivery policy of signals changed in Perl v5.8.0
547               from immediate (also known as "unsafe") to deferred, also known
548               as "safe signals".  See perlipc for more information.
549
550               Certain internal hooks can be also set using the %SIG hash.
551               The routine indicated by $SIG{__WARN__} is called when a
552               warning message is about to be printed.  The warning message is
553               passed as the first argument.  The presence of a "__WARN__"
554               hook causes the ordinary printing of warnings to "STDERR" to be
555               suppressed.  You can use this to save warnings in a variable,
556               or turn warnings into fatal errors, like this:
557
558                   local $SIG{__WARN__} = sub { die $_[0] };
559                   eval $proggie;
560
561               As the 'IGNORE' hook is not supported by "__WARN__", its effect
562               is the same as using 'DEFAULT'.  You can disable warnings using
563               the empty subroutine:
564
565                   local $SIG{__WARN__} = sub {};
566
567               The routine indicated by $SIG{__DIE__} is called when a fatal
568               exception is about to be thrown.  The error message is passed
569               as the first argument.  When a "__DIE__" hook routine returns,
570               the exception processing continues as it would have in the
571               absence of the hook, unless the hook routine itself exits via a
572               "goto &sub", a loop exit, or a "die()".  The "__DIE__" handler
573               is explicitly disabled during the call, so that you can die
574               from a "__DIE__" handler.  Similarly for "__WARN__".
575
576               The $SIG{__DIE__} hook is called even inside an "eval()". It
577               was never intended to happen this way, but an implementation
578               glitch made this possible. This used to be deprecated, as it
579               allowed strange action at a distance like rewriting a pending
580               exception in $@. Plans to rectify this have been scrapped, as
581               users found that rewriting a pending exception is actually a
582               useful feature, and not a bug.
583
584               The $SIG{__DIE__} doesn't support 'IGNORE'; it has the same
585               effect as 'DEFAULT'.
586
587               "__DIE__"/"__WARN__" handlers are very special in one respect:
588               they may be called to report (probable) errors found by the
589               parser.  In such a case the parser may be in inconsistent
590               state, so any attempt to evaluate Perl code from such a handler
591               will probably result in a segfault.  This means that warnings
592               or errors that result from parsing Perl should be used with
593               extreme caution, like this:
594
595                   require Carp if defined $^S;
596                   Carp::confess("Something wrong") if defined &Carp::confess;
597                   die "Something wrong, but could not load Carp to give "
598                     . "backtrace...\n\t"
599                     . "To see backtrace try starting Perl with -MCarp switch";
600
601               Here the first line will load "Carp" unless it is the parser
602               who called the handler.  The second line will print backtrace
603               and die if "Carp" was available.  The third line will be
604               executed only if "Carp" was not available.
605
606               Having to even think about the $^S variable in your exception
607               handlers is simply wrong.  $SIG{__DIE__} as currently
608               implemented invites grievous and difficult to track down
609               errors.  Avoid it and use an "END{}" or CORE::GLOBAL::die
610               override instead.
611
612               See "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc,
613               and warnings for additional information.
614
615       $BASETIME
616       $^T     The time at which the program began running, in seconds since
617               the epoch (beginning of 1970).  The values returned by the -M,
618               -A, and -C filetests are based on this value.
619
620       $PERL_VERSION
621       $^V     The revision, version, and subversion of the Perl interpreter,
622               represented as a version object.
623
624               This variable first appeared in perl v5.6.0; earlier versions
625               of perl will see an undefined value.  Before perl v5.10.0 $^V
626               was represented as a v-string rather than a version object.
627
628               $^V can be used to determine whether the Perl interpreter
629               executing a script is in the right range of versions.  For
630               example:
631
632                   warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
633
634               While version objects overload stringification, to portably
635               convert $^V into its string representation, use "sprintf()"'s
636               "%vd" conversion, which works for both v-strings or version
637               objects:
638
639                   printf "version is v%vd\n", $^V;  # Perl's version
640
641               See the documentation of "use VERSION" and "require VERSION"
642               for a convenient way to fail if the running Perl interpreter is
643               too old.
644
645               See also "$]" for a decimal representation of the Perl version.
646
647               The main advantage of $^V over $] is that, for Perl v5.10.0 or
648               later, it overloads operators, allowing easy comparison against
649               other version representations (e.g. decimal, literal v-string,
650               "v1.2.3", or objects).  The disadvantage is that prior to
651               v5.10.0, it was only a literal v-string, which can't be easily
652               printed or compared, whereas the behavior of $] is unchanged on
653               all versions of Perl.
654
655               Mnemonic: use ^V for a version object.
656
657       ${^WIN32_SLOPPY_STAT}
658               This variable no longer has any function.
659
660               This variable was added in Perl v5.10.0 and removed in Perl
661               v5.34.0.
662
663       $EXECUTABLE_NAME
664       $^X     The name used to execute the current copy of Perl, from C's
665               "argv[0]" or (where supported) /proc/self/exe.
666
667               Depending on the host operating system, the value of $^X may be
668               a relative or absolute pathname of the perl program file, or
669               may be the string used to invoke perl but not the pathname of
670               the perl program file.  Also, most operating systems permit
671               invoking programs that are not in the PATH environment
672               variable, so there is no guarantee that the value of $^X is in
673               PATH.  For VMS, the value may or may not include a version
674               number.
675
676               You usually can use the value of $^X to re-invoke an
677               independent copy of the same perl that is currently running,
678               e.g.,
679
680                   @first_run = `$^X -le "print int rand 100 for 1..100"`;
681
682               But recall that not all operating systems support forking or
683               capturing of the output of commands, so this complex statement
684               may not be portable.
685
686               It is not safe to use the value of $^X as a path name of a
687               file, as some operating systems that have a mandatory suffix on
688               executable files do not require use of the suffix when invoking
689               a command.  To convert the value of $^X to a path name, use the
690               following statements:
691
692                   # Build up a set of file names (not command names).
693                   use Config;
694                   my $this_perl = $^X;
695                   if ($^O ne 'VMS') {
696                       $this_perl .= $Config{_exe}
697                         unless $this_perl =~ m/$Config{_exe}$/i;
698                       }
699
700               Because many operating systems permit anyone with read access
701               to the Perl program file to make a copy of it, patch the copy,
702               and then execute the copy, the security-conscious Perl
703               programmer should take care to invoke the installed copy of
704               perl, not the copy referenced by $^X.  The following statements
705               accomplish this goal, and produce a pathname that can be
706               invoked as a command or referenced as a file.
707
708                   use Config;
709                   my $secure_perl_path = $Config{perlpath};
710                   if ($^O ne 'VMS') {
711                       $secure_perl_path .= $Config{_exe}
712                           unless $secure_perl_path =~ m/$Config{_exe}$/i;
713                       }
714
715   Variables related to regular expressions
716       Most of the special variables related to regular expressions are side
717       effects.  Perl sets these variables when it has a successful match, so
718       you should check the match result before using them.  For instance:
719
720           if( /P(A)TT(ER)N/ ) {
721               print "I found $1 and $2\n";
722               }
723
724       These variables are read-only and dynamically-scoped, unless we note
725       otherwise.
726
727       The dynamic nature of the regular expression variables means that their
728       value is limited to the block that they are in, as demonstrated by this
729       bit of code:
730
731           my $outer = 'Wallace and Grommit';
732           my $inner = 'Mutt and Jeff';
733
734           my $pattern = qr/(\S+) and (\S+)/;
735
736           sub show_n { print "\$1 is $1; \$2 is $2\n" }
737
738           {
739           OUTER:
740               show_n() if $outer =~ m/$pattern/;
741
742               INNER: {
743                   show_n() if $inner =~ m/$pattern/;
744                   }
745
746               show_n();
747           }
748
749       The output shows that while in the "OUTER" block, the values of $1 and
750       $2 are from the match against $outer.  Inside the "INNER" block, the
751       values of $1 and $2 are from the match against $inner, but only until
752       the end of the block (i.e. the dynamic scope).  After the "INNER" block
753       completes, the values of $1 and $2 return to the values for the match
754       against $outer even though we have not made another match:
755
756           $1 is Wallace; $2 is Grommit
757           $1 is Mutt; $2 is Jeff
758           $1 is Wallace; $2 is Grommit
759
760       Performance issues
761
762       Traditionally in Perl, any use of any of the three variables  "$`", $&
763       or "$'" (or their "use English" equivalents) anywhere in the code,
764       caused all subsequent successful pattern matches to make a copy of the
765       matched string, in case the code might subsequently access one of those
766       variables.  This imposed a considerable performance penalty across the
767       whole program, so generally the use of these variables has been
768       discouraged.
769
770       In Perl 5.6.0 the "@-" and "@+" dynamic arrays were introduced that
771       supply the indices of successful matches. So you could for example do
772       this:
773
774           $str =~ /pattern/;
775
776           print $`, $&, $'; # bad: performance hit
777
778           print             # good: no performance hit
779               substr($str, 0,     $-[0]),
780               substr($str, $-[0], $+[0]-$-[0]),
781               substr($str, $+[0]);
782
783       In Perl 5.10.0 the "/p" match operator flag and the "${^PREMATCH}",
784       "${^MATCH}", and "${^POSTMATCH}" variables were introduced, that
785       allowed you to suffer the penalties only on patterns marked with "/p".
786
787       In Perl 5.18.0 onwards, perl started noting the presence of each of the
788       three variables separately, and only copied that part of the string
789       required; so in
790
791           $`; $&; "abcdefgh" =~ /d/
792
793       perl would only copy the "abcd" part of the string. That could make a
794       big difference in something like
795
796           $str = 'x' x 1_000_000;
797           $&; # whoops
798           $str =~ /x/g # one char copied a million times, not a million chars
799
800       In Perl 5.20.0 a new copy-on-write system was enabled by default, which
801       finally fixes all performance issues with these three variables, and
802       makes them safe to use anywhere.
803
804       The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you
805       find uses of these problematic match variables in your code.
806
807       $<digits> ($1, $2, ...)
808               Contains the subpattern from the corresponding set of capturing
809               parentheses from the last successful pattern match, not
810               counting patterns matched in nested blocks that have been
811               exited already.
812
813               Note there is a distinction between a capture buffer which
814               matches the empty string a capture buffer which is optional.
815               Eg, "(x?)" and "(x)?" The latter may be undef, the former not.
816
817               These variables are read-only and dynamically-scoped.
818
819               Mnemonic: like \digits.
820
821       @{^CAPTURE}
822               An array which exposes the contents of the capture buffers, if
823               any, of the last successful pattern match, not counting
824               patterns matched in nested blocks that have been exited
825               already.
826
827               Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1
828               index is equivalent to $2, etc.
829
830                   if ("foal"=~/(.)(.)(.)(.)/) {
831                       print join "-", @{^CAPTURE};
832                   }
833
834               should output "f-o-a-l".
835
836               See also "$<digits> ($1, $2, ...)", "%{^CAPTURE}" and
837               "%{^CAPTURE_ALL}".
838
839               Note that unlike most other regex magic variables there is no
840               single letter equivalent to "@{^CAPTURE}".
841
842               This variable was added in 5.25.7
843
844       $MATCH
845       $&      The string matched by the last successful pattern match (not
846               counting any matches hidden within a BLOCK or "eval()" enclosed
847               by the current BLOCK).
848
849               See "Performance issues" above for the serious performance
850               implications of using this variable (even once) in your code.
851
852               This variable is read-only and dynamically-scoped.
853
854               Mnemonic: like "&" in some editors.
855
856       ${^MATCH}
857               This is similar to $& ($MATCH) except that it does not incur
858               the performance penalty associated with that variable.
859
860               See "Performance issues" above.
861
862               In Perl v5.18 and earlier, it is only guaranteed to return a
863               defined value when the pattern was compiled or executed with
864               the "/p" modifier.  In Perl v5.20, the "/p" modifier does
865               nothing, so "${^MATCH}" does the same thing as $MATCH.
866
867               This variable was added in Perl v5.10.0.
868
869               This variable is read-only and dynamically-scoped.
870
871       $PREMATCH
872       $`      The string preceding whatever was matched by the last
873               successful pattern match, not counting any matches hidden
874               within a BLOCK or "eval" enclosed by the current BLOCK.
875
876               See "Performance issues" above for the serious performance
877               implications of using this variable (even once) in your code.
878
879               This variable is read-only and dynamically-scoped.
880
881               Mnemonic: "`" often precedes a quoted string.
882
883       ${^PREMATCH}
884               This is similar to "$`" ($PREMATCH) except that it does not
885               incur the performance penalty associated with that variable.
886
887               See "Performance issues" above.
888
889               In Perl v5.18 and earlier, it is only guaranteed to return a
890               defined value when the pattern was compiled or executed with
891               the "/p" modifier.  In Perl v5.20, the "/p" modifier does
892               nothing, so "${^PREMATCH}" does the same thing as $PREMATCH.
893
894               This variable was added in Perl v5.10.0.
895
896               This variable is read-only and dynamically-scoped.
897
898       $POSTMATCH
899       $'      The string following whatever was matched by the last
900               successful pattern match (not counting any matches hidden
901               within a BLOCK or "eval()" enclosed by the current BLOCK).
902               Example:
903
904                   local $_ = 'abcdefghi';
905                   /def/;
906                   print "$`:$&:$'\n";         # prints abc:def:ghi
907
908               See "Performance issues" above for the serious performance
909               implications of using this variable (even once) in your code.
910
911               This variable is read-only and dynamically-scoped.
912
913               Mnemonic: "'" often follows a quoted string.
914
915       ${^POSTMATCH}
916               This is similar to "$'" ($POSTMATCH) except that it does not
917               incur the performance penalty associated with that variable.
918
919               See "Performance issues" above.
920
921               In Perl v5.18 and earlier, it is only guaranteed to return a
922               defined value when the pattern was compiled or executed with
923               the "/p" modifier.  In Perl v5.20, the "/p" modifier does
924               nothing, so "${^POSTMATCH}" does the same thing as $POSTMATCH.
925
926               This variable was added in Perl v5.10.0.
927
928               This variable is read-only and dynamically-scoped.
929
930       $LAST_PAREN_MATCH
931       $+      The text matched by the highest used capture group of the last
932               successful search pattern.  It is logically equivalent to the
933               highest numbered capture variable ($1, $2, ...) which has a
934               defined value.
935
936               This is useful if you don't know which one of a set of
937               alternative patterns matched.  For example:
938
939                   /Version: (.*)|Revision: (.*)/ && ($rev = $+);
940
941               This variable is read-only and dynamically-scoped.
942
943               Mnemonic: be positive and forward looking.
944
945       $LAST_SUBMATCH_RESULT
946       $^N     The text matched by the used group most-recently closed (i.e.
947               the group with the rightmost closing parenthesis) of the last
948               successful search pattern. This is subtly different from $+.
949               For example in
950
951                   "ab" =~ /^((.)(.))$/
952
953               we have
954
955                   $1,$^N   have the value "ab"
956                   $2       has  the value "a"
957                   $3,$+    have the value "b"
958
959               This is primarily used inside "(?{...})" blocks for examining
960               text recently matched.  For example, to effectively capture
961               text to a variable (in addition to $1, $2, etc.), replace
962               "(...)" with
963
964                   (?:(...)(?{ $var = $^N }))
965
966               By setting and then using $var in this way relieves you from
967               having to worry about exactly which numbered set of parentheses
968               they are.
969
970               This variable was added in Perl v5.8.0.
971
972               Mnemonic: the (possibly) Nested parenthesis that most recently
973               closed.
974
975       @LAST_MATCH_END
976       @+      This array holds the offsets of the ends of the last successful
977               submatches in the currently active dynamic scope.  $+[0] is the
978               offset into the string of the end of the entire match.  This is
979               the same value as what the "pos" function returns when called
980               on the variable that was matched against.  The nth element of
981               this array holds the offset of the nth submatch, so $+[1] is
982               the offset past where $1 ends, $+[2] the offset past where $2
983               ends, and so on.  You can use $#+ to determine how many
984               subgroups were in the last successful match.  See the examples
985               given for the "@-" variable.
986
987               This variable was added in Perl v5.6.0.
988
989       %{^CAPTURE}
990       %LAST_PAREN_MATCH
991       %+      Similar to "@+", the "%+" hash allows access to the named
992               capture buffers, should they exist, in the last successful
993               match in the currently active dynamic scope.
994
995               For example, $+{foo} is equivalent to $1 after the following
996               match:
997
998                   'foo' =~ /(?<foo>foo)/;
999
1000               The keys of the "%+" hash list only the names of buffers that
1001               have captured (and that are thus associated to defined values).
1002
1003               If multiple distinct capture groups have the same name, then
1004               $+{NAME} will refer to the leftmost defined group in the match.
1005
1006               The underlying behaviour of "%+" is provided by the
1007               Tie::Hash::NamedCapture module.
1008
1009               Note: "%-" and "%+" are tied views into a common internal hash
1010               associated with the last successful regular expression.
1011               Therefore mixing iterative access to them via "each" may have
1012               unpredictable results.  Likewise, if the last successful match
1013               changes, then the results may be surprising.
1014
1015               This variable was added in Perl v5.10.0. The "%{^CAPTURE}"
1016               alias was added in 5.25.7.
1017
1018               This variable is read-only and dynamically-scoped.
1019
1020       @LAST_MATCH_START
1021       @-      "$-[0]" is the offset of the start of the last successful
1022               match.  "$-[n]" is the offset of the start of the substring
1023               matched by n-th subpattern, or undef if the subpattern did not
1024               match.
1025
1026               Thus, after a match against $_, $& coincides with "substr $_,
1027               $-[0], $+[0] - $-[0]".  Similarly, $n coincides with "substr
1028               $_, $-[n], $+[n] - $-[n]" if "$-[n]" is defined, and $+
1029               coincides with "substr $_, $-[$#-], $+[$#-] - $-[$#-]".  One
1030               can use "$#-" to find the last matched subgroup in the last
1031               successful match.  Contrast with $#+, the number of subgroups
1032               in the regular expression.  Compare with "@+".
1033
1034               This array holds the offsets of the beginnings of the last
1035               successful submatches in the currently active dynamic scope.
1036               "$-[0]" is the offset into the string of the beginning of the
1037               entire match.  The nth element of this array holds the offset
1038               of the nth submatch, so "$-[1]" is the offset where $1 begins,
1039               "$-[2]" the offset where $2 begins, and so on.
1040
1041               After a match against some variable $var:
1042
1043               "$`" is the same as "substr($var, 0, $-[0])"
1044               $& is the same as "substr($var, $-[0], $+[0] - $-[0])"
1045               "$'" is the same as "substr($var, $+[0])"
1046               $1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
1047               $2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
1048               $3 is the same as "substr($var, $-[3], $+[3] - $-[3])"
1049
1050               This variable was added in Perl v5.6.0.
1051
1052       %{^CAPTURE_ALL}
1053       %-      Similar to "%+", this variable allows access to the named
1054               capture groups in the last successful match in the currently
1055               active dynamic scope.  To each capture group name found in the
1056               regular expression, it associates a reference to an array
1057               containing the list of values captured by all buffers with that
1058               name (should there be several of them), in the order where they
1059               appear.
1060
1061               Here's an example:
1062
1063                   if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1064                       foreach my $bufname (sort keys %-) {
1065                           my $ary = $-{$bufname};
1066                           foreach my $idx (0..$#$ary) {
1067                               print "\$-{$bufname}[$idx] : ",
1068                                     (defined($ary->[$idx])
1069                                         ? "'$ary->[$idx]'"
1070                                         : "undef"),
1071                                     "\n";
1072                           }
1073                       }
1074                   }
1075
1076               would print out:
1077
1078                   $-{A}[0] : '1'
1079                   $-{A}[1] : '3'
1080                   $-{B}[0] : '2'
1081                   $-{B}[1] : '4'
1082
1083               The keys of the "%-" hash correspond to all buffer names found
1084               in the regular expression.
1085
1086               The behaviour of "%-" is implemented via the
1087               Tie::Hash::NamedCapture module.
1088
1089               Note: "%-" and "%+" are tied views into a common internal hash
1090               associated with the last successful regular expression.
1091               Therefore mixing iterative access to them via "each" may have
1092               unpredictable results.  Likewise, if the last successful match
1093               changes, then the results may be surprising.
1094
1095               This variable was added in Perl v5.10.0. The "%{^CAPTURE_ALL}"
1096               alias was added in 5.25.7.
1097
1098               This variable is read-only and dynamically-scoped.
1099
1100       $LAST_REGEXP_CODE_RESULT
1101       $^R     The result of evaluation of the last successful "(?{ code })"
1102               regular expression assertion (see perlre).  May be written to.
1103
1104               This variable was added in Perl 5.005.
1105
1106       ${^RE_COMPILE_RECURSION_LIMIT}
1107               The current value giving the maximum number of open but
1108               unclosed parenthetical groups there may be at any point during
1109               a regular expression compilation.  The default is currently
1110               1000 nested groups.  You may adjust it depending on your needs
1111               and the amount of memory available.
1112
1113               This variable was added in Perl v5.30.0.
1114
1115       ${^RE_DEBUG_FLAGS}
1116               The current value of the regex debugging flags.  Set to 0 for
1117               no debug output even when the "re 'debug'" module is loaded.
1118               See re for details.
1119
1120               This variable was added in Perl v5.10.0.
1121
1122       ${^RE_TRIE_MAXBUF}
1123               Controls how certain regex optimisations are applied and how
1124               much memory they utilize.  This value by default is 65536 which
1125               corresponds to a 512kB temporary cache.  Set this to a higher
1126               value to trade memory for speed when matching large
1127               alternations.  Set it to a lower value if you want the
1128               optimisations to be as conservative of memory as possible but
1129               still occur, and set it to a negative value to prevent the
1130               optimisation and conserve the most memory.  Under normal
1131               situations this variable should be of no interest to you.
1132
1133               This variable was added in Perl v5.10.0.
1134
1135   Variables related to filehandles
1136       Variables that depend on the currently selected filehandle may be set
1137       by calling an appropriate object method on the "IO::Handle" object,
1138       although this is less efficient than using the regular built-in
1139       variables.  (Summary lines below for this contain the word HANDLE.)
1140       First you must say
1141
1142           use IO::Handle;
1143
1144       after which you may use either
1145
1146           method HANDLE EXPR
1147
1148       or more safely,
1149
1150           HANDLE->method(EXPR)
1151
1152       Each method returns the old value of the "IO::Handle" attribute.  The
1153       methods each take an optional EXPR, which, if supplied, specifies the
1154       new value for the "IO::Handle" attribute in question.  If not supplied,
1155       most methods do nothing to the current value--except for "autoflush()",
1156       which will assume a 1 for you, just to be different.
1157
1158       Because loading in the "IO::Handle" class is an expensive operation,
1159       you should learn how to use the regular built-in variables.
1160
1161       A few of these variables are considered "read-only".  This means that
1162       if you try to assign to this variable, either directly or indirectly
1163       through a reference, you'll raise a run-time exception.
1164
1165       You should be very careful when modifying the default values of most
1166       special variables described in this document.  In most cases you want
1167       to localize these variables before changing them, since if you don't,
1168       the change may affect other modules which rely on the default values of
1169       the special variables that you have changed.  This is one of the
1170       correct ways to read the whole file at once:
1171
1172           open my $fh, "<", "foo" or die $!;
1173           local $/; # enable localized slurp mode
1174           my $content = <$fh>;
1175           close $fh;
1176
1177       But the following code is quite bad:
1178
1179           open my $fh, "<", "foo" or die $!;
1180           undef $/; # enable slurp mode
1181           my $content = <$fh>;
1182           close $fh;
1183
1184       since some other module, may want to read data from some file in the
1185       default "line mode", so if the code we have just presented has been
1186       executed, the global value of $/ is now changed for any other code
1187       running inside the same Perl interpreter.
1188
1189       Usually when a variable is localized you want to make sure that this
1190       change affects the shortest scope possible.  So unless you are already
1191       inside some short "{}" block, you should create one yourself.  For
1192       example:
1193
1194           my $content = '';
1195           open my $fh, "<", "foo" or die $!;
1196           {
1197               local $/;
1198               $content = <$fh>;
1199           }
1200           close $fh;
1201
1202       Here is an example of how your own code can go broken:
1203
1204           for ( 1..3 ){
1205               $\ = "\r\n";
1206               nasty_break();
1207               print "$_";
1208           }
1209
1210           sub nasty_break {
1211               $\ = "\f";
1212               # do something with $_
1213           }
1214
1215       You probably expect this code to print the equivalent of
1216
1217           "1\r\n2\r\n3\r\n"
1218
1219       but instead you get:
1220
1221           "1\f2\f3\f"
1222
1223       Why? Because "nasty_break()" modifies "$\" without localizing it first.
1224       The value you set in  "nasty_break()" is still there when you return.
1225       The fix is to add "local()" so the value doesn't leak out of
1226       "nasty_break()":
1227
1228           local $\ = "\f";
1229
1230       It's easy to notice the problem in such a short example, but in more
1231       complicated code you are looking for trouble if you don't localize
1232       changes to the special variables.
1233
1234       $ARGV   Contains the name of the current file when reading from "<>".
1235
1236       @ARGV   The array @ARGV contains the command-line arguments intended
1237               for the script.  $#ARGV is generally the number of arguments
1238               minus one, because $ARGV[0] is the first argument, not the
1239               program's command name itself.  See "$0" for the command name.
1240
1241       ARGV    The special filehandle that iterates over command-line
1242               filenames in @ARGV.  Usually written as the null filehandle in
1243               the angle operator "<>".  Note that currently "ARGV" only has
1244               its magical effect within the "<>" operator; elsewhere it is
1245               just a plain filehandle corresponding to the last file opened
1246               by "<>".  In particular, passing "\*ARGV" as a parameter to a
1247               function that expects a filehandle may not cause your function
1248               to automatically read the contents of all the files in @ARGV.
1249
1250       ARGVOUT The special filehandle that points to the currently open output
1251               file when doing edit-in-place processing with -i.  Useful when
1252               you have to do a lot of inserting and don't want to keep
1253               modifying $_.  See perlrun for the -i switch.
1254
1255       IO::Handle->output_field_separator( EXPR )
1256       $OUTPUT_FIELD_SEPARATOR
1257       $OFS
1258       $,      The output field separator for the print operator.  If defined,
1259               this value is printed between each of print's arguments.
1260               Default is "undef".
1261
1262               You cannot call "output_field_separator()" on a handle, only as
1263               a static method.  See IO::Handle.
1264
1265               Mnemonic: what is printed when there is a "," in your print
1266               statement.
1267
1268       HANDLE->input_line_number( EXPR )
1269       $INPUT_LINE_NUMBER
1270       $NR
1271       $.      Current line number for the last filehandle accessed.
1272
1273               Each filehandle in Perl counts the number of lines that have
1274               been read from it.  (Depending on the value of $/, Perl's idea
1275               of what constitutes a line may not match yours.)  When a line
1276               is read from a filehandle (via "readline()" or "<>"), or when
1277               "tell()" or "seek()" is called on it, $. becomes an alias to
1278               the line counter for that filehandle.
1279
1280               You can adjust the counter by assigning to $., but this will
1281               not actually move the seek pointer.  Localizing $. will not
1282               localize the filehandle's line count.  Instead, it will
1283               localize perl's notion of which filehandle $. is currently
1284               aliased to.
1285
1286               $. is reset when the filehandle is closed, but not when an open
1287               filehandle is reopened without an intervening "close()".  For
1288               more details, see "I/O Operators" in perlop.  Because "<>"
1289               never does an explicit close, line numbers increase across
1290               "ARGV" files (but see examples in "eof" in perlfunc).
1291
1292               You can also use "HANDLE->input_line_number(EXPR)" to access
1293               the line counter for a given filehandle without having to worry
1294               about which handle you last accessed.
1295
1296               Mnemonic: many programs use "." to mean the current line
1297               number.
1298
1299       IO::Handle->input_record_separator( EXPR )
1300       $INPUT_RECORD_SEPARATOR
1301       $RS
1302       $/      The input record separator, newline by default.  This
1303               influences Perl's idea of what a "line" is.  Works like awk's
1304               RS variable, including treating empty lines as a terminator if
1305               set to the null string (an empty line cannot contain any spaces
1306               or tabs).  You may set it to a multi-character string to match
1307               a multi-character terminator, or to "undef" to read through the
1308               end of file.  Setting it to "\n\n" means something slightly
1309               different than setting to "", if the file contains consecutive
1310               empty lines.  Setting to "" will treat two or more consecutive
1311               empty lines as a single empty line.  Setting to "\n\n" will
1312               blindly assume that the next input character belongs to the
1313               next paragraph, even if it's a newline.
1314
1315                   local $/;           # enable "slurp" mode
1316                   local $_ = <FH>;    # whole file now here
1317                   s/\n[ \t]+/ /g;
1318
1319               Remember: the value of $/ is a string, not a regex.  awk has to
1320               be better for something. :-)
1321
1322               Setting $/ to an empty string -- the so-called paragraph mode
1323               -- merits special attention.  When $/ is set to "" and the
1324               entire file is read in with that setting, any sequence of one
1325               or more consecutive newlines at the beginning of the file is
1326               discarded.  With the exception of the final record in the file,
1327               each sequence of characters ending in two or more newlines is
1328               treated as one record and is read in to end in exactly two
1329               newlines.  If the last record in the file ends in zero or one
1330               consecutive newlines, that record is read in with that number
1331               of newlines.  If the last record ends in two or more
1332               consecutive newlines, it is read in with two newlines like all
1333               preceding records.
1334
1335               Suppose we wrote the following string to a file:
1336
1337                   my $string = "\n\n\n";
1338                   $string .= "alpha beta\ngamma delta\n\n\n";
1339                   $string .= "epsilon zeta eta\n\n";
1340                   $string .= "theta\n";
1341
1342                   my $file = 'simple_file.txt';
1343                   open my $OUT, '>', $file or die;
1344                   print $OUT $string;
1345                   close $OUT or die;
1346
1347               Now we read that file in paragraph mode:
1348
1349                   local $/ = ""; # paragraph mode
1350                   open my $IN, '<', $file or die;
1351                   my @records = <$IN>;
1352                   close $IN or die;
1353
1354               @records will consist of these 3 strings:
1355
1356                   (
1357                     "alpha beta\ngamma delta\n\n",
1358                     "epsilon zeta eta\n\n",
1359                     "theta\n",
1360                   )
1361
1362               Setting $/ to a reference to an integer, scalar containing an
1363               integer, or scalar that's convertible to an integer will
1364               attempt to read records instead of lines, with the maximum
1365               record size being the referenced integer number of characters.
1366               So this:
1367
1368                   local $/ = \32768; # or \"32768", or \$var_containing_32768
1369                   open my $fh, "<", $myfile or die $!;
1370                   local $_ = <$fh>;
1371
1372               will read a record of no more than 32768 characters from $fh.
1373               If you're not reading from a record-oriented file (or your OS
1374               doesn't have record-oriented files), then you'll likely get a
1375               full chunk of data with every read.  If a record is larger than
1376               the record size you've set, you'll get the record back in
1377               pieces.  Trying to set the record size to zero or less is
1378               deprecated and will cause $/ to have the value of "undef",
1379               which will cause reading in the (rest of the) whole file.
1380
1381               As of 5.19.9 setting $/ to any other form of reference will
1382               throw a fatal exception. This is in preparation for supporting
1383               new ways to set $/ in the future.
1384
1385               On VMS only, record reads bypass PerlIO layers and any
1386               associated buffering, so you must not mix record and non-record
1387               reads on the same filehandle.  Record mode mixes with line mode
1388               only when the same buffering layer is in use for both modes.
1389
1390               You cannot call "input_record_separator()" on a handle, only as
1391               a static method.  See IO::Handle.
1392
1393               See also "Newlines" in perlport.  Also see "$.".
1394
1395               Mnemonic: / delimits line boundaries when quoting poetry.
1396
1397       IO::Handle->output_record_separator( EXPR )
1398       $OUTPUT_RECORD_SEPARATOR
1399       $ORS
1400       $\      The output record separator for the print operator.  If
1401               defined, this value is printed after the last of print's
1402               arguments.  Default is "undef".
1403
1404               You cannot call "output_record_separator()" on a handle, only
1405               as a static method.  See IO::Handle.
1406
1407               Mnemonic: you set "$\" instead of adding "\n" at the end of the
1408               print.  Also, it's just like $/, but it's what you get "back"
1409               from Perl.
1410
1411       HANDLE->autoflush( EXPR )
1412       $OUTPUT_AUTOFLUSH
1413       $|      If set to nonzero, forces a flush right away and after every
1414               write or print on the currently selected output channel.
1415               Default is 0 (regardless of whether the channel is really
1416               buffered by the system or not; $| tells you only whether you've
1417               asked Perl explicitly to flush after each write).  STDOUT will
1418               typically be line buffered if output is to the terminal and
1419               block buffered otherwise.  Setting this variable is useful
1420               primarily when you are outputting to a pipe or socket, such as
1421               when you are running a Perl program under rsh and want to see
1422               the output as it's happening.  This has no effect on input
1423               buffering.  See "getc" in perlfunc for that.  See "select" in
1424               perlfunc on how to select the output channel.  See also
1425               IO::Handle.
1426
1427               Mnemonic: when you want your pipes to be piping hot.
1428
1429       ${^LAST_FH}
1430               This read-only variable contains a reference to the last-read
1431               filehandle.  This is set by "<HANDLE>", "readline", "tell",
1432               "eof" and "seek".  This is the same handle that $. and "tell"
1433               and "eof" without arguments use.  It is also the handle used
1434               when Perl appends ", <STDIN> line 1" to an error or warning
1435               message.
1436
1437               This variable was added in Perl v5.18.0.
1438
1439       Variables related to formats
1440
1441       The special variables for formats are a subset of those for
1442       filehandles.  See perlform for more information about Perl's formats.
1443
1444       $ACCUMULATOR
1445       $^A     The current value of the "write()" accumulator for "format()"
1446               lines.  A format contains "formline()" calls that put their
1447               result into $^A.  After calling its format, "write()" prints
1448               out the contents of $^A and empties.  So you never really see
1449               the contents of $^A unless you call "formline()" yourself and
1450               then look at it.  See perlform and "formline PICTURE,LIST" in
1451               perlfunc.
1452
1453       IO::Handle->format_formfeed(EXPR)
1454       $FORMAT_FORMFEED
1455       $^L     What formats output as a form feed.  The default is "\f".
1456
1457               You cannot call "format_formfeed()" on a handle, only as a
1458               static method.  See IO::Handle.
1459
1460       HANDLE->format_page_number(EXPR)
1461       $FORMAT_PAGE_NUMBER
1462       $%      The current page number of the currently selected output
1463               channel.
1464
1465               Mnemonic: "%" is page number in nroff.
1466
1467       HANDLE->format_lines_left(EXPR)
1468       $FORMAT_LINES_LEFT
1469       $-      The number of lines left on the page of the currently selected
1470               output channel.
1471
1472               Mnemonic: lines_on_page - lines_printed.
1473
1474       IO::Handle->format_line_break_characters EXPR
1475       $FORMAT_LINE_BREAK_CHARACTERS
1476       $:      The current set of characters after which a string may be
1477               broken to fill continuation fields (starting with "^") in a
1478               format.  The default is " \n-", to break on a space, newline,
1479               or a hyphen.
1480
1481               You cannot call "format_line_break_characters()" on a handle,
1482               only as a static method.  See IO::Handle.
1483
1484               Mnemonic: a "colon" in poetry is a part of a line.
1485
1486       HANDLE->format_lines_per_page(EXPR)
1487       $FORMAT_LINES_PER_PAGE
1488       $=      The current page length (printable lines) of the currently
1489               selected output channel.  The default is 60.
1490
1491               Mnemonic: = has horizontal lines.
1492
1493       HANDLE->format_top_name(EXPR)
1494       $FORMAT_TOP_NAME
1495       $^      The name of the current top-of-page format for the currently
1496               selected output channel.  The default is the name of the
1497               filehandle with "_TOP" appended.  For example, the default
1498               format top name for the "STDOUT" filehandle is "STDOUT_TOP".
1499
1500               Mnemonic: points to top of page.
1501
1502       HANDLE->format_name(EXPR)
1503       $FORMAT_NAME
1504       $~      The name of the current report format for the currently
1505               selected output channel.  The default format name is the same
1506               as the filehandle name.  For example, the default format name
1507               for the "STDOUT" filehandle is just "STDOUT".
1508
1509               Mnemonic: brother to $^.
1510
1511   Error Variables
1512       The variables $@, $!, $^E, and $? contain information about different
1513       types of error conditions that may appear during execution of a Perl
1514       program.  The variables are shown ordered by the "distance" between the
1515       subsystem which reported the error and the Perl process.  They
1516       correspond to errors detected by the Perl interpreter, C library,
1517       operating system, or an external program, respectively.
1518
1519       To illustrate the differences between these variables, consider the
1520       following Perl expression, which uses a single-quoted string.  After
1521       execution of this statement, perl may have set all four special error
1522       variables:
1523
1524           eval q{
1525               open my $pipe, "/cdrom/install |" or die $!;
1526               my @res = <$pipe>;
1527               close $pipe or die "bad pipe: $?, $!";
1528           };
1529
1530       When perl executes the "eval()" expression, it translates the "open()",
1531       "<PIPE>", and "close" calls in the C run-time library and thence to the
1532       operating system kernel.  perl sets $! to the C library's "errno" if
1533       one of these calls fails.
1534
1535       $@ is set if the string to be "eval"-ed did not compile (this may
1536       happen if "open" or "close" were imported with bad prototypes), or if
1537       Perl code executed during evaluation "die()"d.  In these cases the
1538       value of $@ is the compile error, or the argument to "die" (which will
1539       interpolate $! and $?).  (See also Fatal, though.)
1540
1541       Under a few operating systems, $^E may contain a more verbose error
1542       indicator, such as in this case, "CDROM tray not closed."  Systems that
1543       do not support extended error messages leave $^E the same as $!.
1544
1545       Finally, $? may be set to a non-0 value if the external program
1546       /cdrom/install fails.  The upper eight bits reflect specific error
1547       conditions encountered by the program (the program's "exit()" value).
1548       The lower eight bits reflect mode of failure, like signal death and
1549       core dump information.  See wait(2) for details.  In contrast to $! and
1550       $^E, which are set only if an error condition is detected, the variable
1551       $? is set on each "wait" or pipe "close", overwriting the old value.
1552       This is more like $@, which on every "eval()" is always set on failure
1553       and cleared on success.
1554
1555       For more details, see the individual descriptions at $@, $!, $^E, and
1556       $?.
1557
1558       ${^CHILD_ERROR_NATIVE}
1559               The native status returned by the last pipe close, backtick
1560               ("``") command, successful call to "wait()" or "waitpid()", or
1561               from the "system()" operator.  On POSIX-like systems this value
1562               can be decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED,
1563               WTERMSIG, WIFSTOPPED, and WSTOPSIG functions provided by the
1564               POSIX module.
1565
1566               Under VMS this reflects the actual VMS exit status; i.e. it is
1567               the same as $? when the pragma "use vmsish 'status'" is in
1568               effect.
1569
1570               This variable was added in Perl v5.10.0.
1571
1572       $EXTENDED_OS_ERROR
1573       $^E     Error information specific to the current operating system.  At
1574               the moment, this differs from "$!" under only VMS, OS/2, and
1575               Win32 (and for MacPerl).  On all other platforms, $^E is always
1576               just the same as $!.
1577
1578               Under VMS, $^E provides the VMS status value from the last
1579               system error.  This is more specific information about the last
1580               system error than that provided by $!.  This is particularly
1581               important when $!  is set to EVMSERR.
1582
1583               Under OS/2, $^E is set to the error code of the last call to
1584               OS/2 API either via CRT, or directly from perl.
1585
1586               Under Win32, $^E always returns the last error information
1587               reported by the Win32 call "GetLastError()" which describes the
1588               last error from within the Win32 API.  Most Win32-specific code
1589               will report errors via $^E.  ANSI C and Unix-like calls set
1590               "errno" and so most portable Perl code will report errors via
1591               $!.
1592
1593               Caveats mentioned in the description of "$!" generally apply to
1594               $^E, also.
1595
1596               This variable was added in Perl 5.003.
1597
1598               Mnemonic: Extra error explanation.
1599
1600       $EXCEPTIONS_BEING_CAUGHT
1601       $^S     Current state of the interpreter.
1602
1603                       $^S         State
1604                       ---------   -------------------------------------
1605                       undef       Parsing module, eval, or main program
1606                       true (1)    Executing an eval
1607                       false (0)   Otherwise
1608
1609               The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
1610               handlers.
1611
1612               The English name $EXCEPTIONS_BEING_CAUGHT is slightly
1613               misleading, because the "undef" value does not indicate whether
1614               exceptions are being caught, since compilation of the main
1615               program does not catch exceptions.
1616
1617               This variable was added in Perl 5.004.
1618
1619       $WARNING
1620       $^W     The current value of the warning switch, initially true if -w
1621               was used, false otherwise, but directly modifiable.
1622
1623               See also warnings.
1624
1625               Mnemonic: related to the -w switch.
1626
1627       ${^WARNING_BITS}
1628               The current set of warning checks enabled by the "use warnings"
1629               pragma.  It has the same scoping as the $^H and "%^H"
1630               variables.  The exact values are considered internal to the
1631               warnings pragma and may change between versions of Perl.
1632
1633               This variable was added in Perl v5.6.0.
1634
1635       $OS_ERROR
1636       $ERRNO
1637       $!      When referenced, $! retrieves the current value of the C
1638               "errno" integer variable.  If $! is assigned a numerical value,
1639               that value is stored in "errno".  When referenced as a string,
1640               $! yields the system error string corresponding to "errno".
1641
1642               Many system or library calls set "errno" if they fail, to
1643               indicate the cause of failure.  They usually do not set "errno"
1644               to zero if they succeed and may set "errno" to a non-zero value
1645               on success.  This means "errno", hence $!, is meaningful only
1646               immediately after a failure:
1647
1648                   if (open my $fh, "<", $filename) {
1649                               # Here $! is meaningless.
1650                               ...
1651                   }
1652                   else {
1653                               # ONLY here is $! meaningful.
1654                               ...
1655                               # Already here $! might be meaningless.
1656                   }
1657                   # Since here we might have either success or failure,
1658                   # $! is meaningless.
1659
1660               Here, meaningless means that $! may be unrelated to the outcome
1661               of the "open()" operator.  Assignment to $! is similarly
1662               ephemeral.  It can be used immediately before invoking the
1663               "die()" operator, to set the exit value, or to inspect the
1664               system error string corresponding to error n, or to restore $!
1665               to a meaningful state.
1666
1667               Perl itself may set "errno" to a non-zero on failure even if no
1668               system call is performed.
1669
1670               Mnemonic: What just went bang?
1671
1672       %OS_ERROR
1673       %ERRNO
1674       %!      Each element of "%!" has a true value only if $! is set to that
1675               value.  For example, $!{ENOENT} is true if and only if the
1676               current value of $! is "ENOENT"; that is, if the most recent
1677               error was "No such file or directory" (or its moral equivalent:
1678               not all operating systems give that exact error, and certainly
1679               not all languages).  The specific true value is not guaranteed,
1680               but in the past has generally been the numeric value of $!.  To
1681               check if a particular key is meaningful on your system, use
1682               "exists $!{the_key}"; for a list of legal keys, use "keys %!".
1683               See Errno for more information, and also see "$!".
1684
1685               This variable was added in Perl 5.005.
1686
1687       $CHILD_ERROR
1688       $?      The status returned by the last pipe close, backtick ("``")
1689               command, successful call to "wait()" or "waitpid()", or from
1690               the "system()" operator.  This is just the 16-bit status word
1691               returned by the traditional Unix "wait()" system call (or else
1692               is made up to look like it).  Thus, the exit value of the
1693               subprocess is really ("$? >> 8"), and "$? & 127" gives which
1694               signal, if any, the process died from, and "$? & 128" reports
1695               whether there was a core dump.
1696
1697               Additionally, if the "h_errno" variable is supported in C, its
1698               value is returned via $? if any "gethost*()" function fails.
1699
1700               If you have installed a signal handler for "SIGCHLD", the value
1701               of $? will usually be wrong outside that handler.
1702
1703               Inside an "END" subroutine $? contains the value that is going
1704               to be given to "exit()".  You can modify $? in an "END"
1705               subroutine to change the exit status of your program.  For
1706               example:
1707
1708                   END {
1709                       $? = 1 if $? == 255;  # die would make it 255
1710                   }
1711
1712               Under VMS, the pragma "use vmsish 'status'" makes $? reflect
1713               the actual VMS exit status, instead of the default emulation of
1714               POSIX status; see "$?" in perlvms for details.
1715
1716               Mnemonic: similar to sh and ksh.
1717
1718       $EVAL_ERROR
1719       $@      The Perl error from the last "eval" operator, i.e. the last
1720               exception that was caught.  For "eval BLOCK", this is either a
1721               runtime error message or the string or reference "die" was
1722               called with.  The "eval STRING" form also catches syntax errors
1723               and other compile time exceptions.
1724
1725               If no error occurs, "eval" sets $@ to the empty string.
1726
1727               Warning messages are not collected in this variable.  You can,
1728               however, set up a routine to process warnings by setting
1729               $SIG{__WARN__} as described in "%SIG".
1730
1731               Mnemonic: Where was the error "at"?
1732
1733   Variables related to the interpreter state
1734       These variables provide information about the current interpreter
1735       state.
1736
1737       $COMPILING
1738       $^C     The current value of the flag associated with the -c switch.
1739               Mainly of use with -MO=... to allow code to alter its behavior
1740               when being compiled, such as for example to "AUTOLOAD" at
1741               compile time rather than normal, deferred loading.  Setting
1742               "$^C = 1" is similar to calling "B::minus_c".
1743
1744               This variable was added in Perl v5.6.0.
1745
1746       $DEBUGGING
1747       $^D     The current value of the debugging flags.  May be read or set.
1748               Like its command-line equivalent, you can use numeric or
1749               symbolic values, e.g. "$^D = 10" or "$^D = "st"".  See
1750               "-Dnumber" in perlrun.  The contents of this variable also
1751               affects the debugger operation.  See "Debugger Internals" in
1752               perldebguts.
1753
1754               Mnemonic: value of -D switch.
1755
1756       ${^ENCODING}
1757               This variable is no longer supported.
1758
1759               It used to hold the object reference to the "Encode" object
1760               that was used to convert the source code to Unicode.
1761
1762               Its purpose was to allow your non-ASCII Perl scripts not to
1763               have to be written in UTF-8; this was useful before editors
1764               that worked on UTF-8 encoded text were common, but that was
1765               long ago.  It caused problems, such as affecting the operation
1766               of other modules that weren't expecting it, causing general
1767               mayhem.
1768
1769               If you need something like this functionality, it is
1770               recommended that use you a simple source filter, such as
1771               Filter::Encoding.
1772
1773               If you are coming here because code of yours is being adversely
1774               affected by someone's use of this variable, you can usually
1775               work around it by doing this:
1776
1777                local ${^ENCODING};
1778
1779               near the beginning of the functions that are getting broken.
1780               This undefines the variable during the scope of execution of
1781               the including function.
1782
1783               This variable was added in Perl 5.8.2 and removed in 5.26.0.
1784               Setting it to anything other than "undef" was made fatal in
1785               Perl 5.28.0.
1786
1787       ${^GLOBAL_PHASE}
1788               The current phase of the perl interpreter.
1789
1790               Possible values are:
1791
1792               CONSTRUCT
1793                       The "PerlInterpreter*" is being constructed via
1794                       "perl_construct".  This value is mostly there for
1795                       completeness and for use via the underlying C variable
1796                       "PL_phase".  It's not really possible for Perl code to
1797                       be executed unless construction of the interpreter is
1798                       finished.
1799
1800               START   This is the global compile-time.  That includes,
1801                       basically, every "BEGIN" block executed directly or
1802                       indirectly from during the compile-time of the top-
1803                       level program.
1804
1805                       This phase is not called "BEGIN" to avoid confusion
1806                       with "BEGIN"-blocks, as those are executed during
1807                       compile-time of any compilation unit, not just the top-
1808                       level program.  A new, localised compile-time entered
1809                       at run-time, for example by constructs as "eval "use
1810                       SomeModule"" are not global interpreter phases, and
1811                       therefore aren't reflected by "${^GLOBAL_PHASE}".
1812
1813               CHECK   Execution of any "CHECK" blocks.
1814
1815               INIT    Similar to "CHECK", but for "INIT"-blocks, not "CHECK"
1816                       blocks.
1817
1818               RUN     The main run-time, i.e. the execution of
1819                       "PL_main_root".
1820
1821               END     Execution of any "END" blocks.
1822
1823               DESTRUCT
1824                       Global destruction.
1825
1826               Also note that there's no value for UNITCHECK-blocks.  That's
1827               because those are run for each compilation unit individually,
1828               and therefore is not a global interpreter phase.
1829
1830               Not every program has to go through each of the possible
1831               phases, but transition from one phase to another can only
1832               happen in the order described in the above list.
1833
1834               An example of all of the phases Perl code can see:
1835
1836                   BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1837
1838                   INIT  { print "init-time: ${^GLOBAL_PHASE}\n" }
1839
1840                   CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1841
1842                   {
1843                       package Print::Phase;
1844
1845                       sub new {
1846                           my ($class, $time) = @_;
1847                           return bless \$time, $class;
1848                       }
1849
1850                       sub DESTROY {
1851                           my $self = shift;
1852                           print "$$self: ${^GLOBAL_PHASE}\n";
1853                       }
1854                   }
1855
1856                   print "run-time: ${^GLOBAL_PHASE}\n";
1857
1858                   my $runtime = Print::Phase->new(
1859                       "lexical variables are garbage collected before END"
1860                   );
1861
1862                   END   { print "end-time: ${^GLOBAL_PHASE}\n" }
1863
1864                   our $destruct = Print::Phase->new(
1865                       "package variables are garbage collected after END"
1866                   );
1867
1868               This will print out
1869
1870                   compile-time: START
1871                   check-time: CHECK
1872                   init-time: INIT
1873                   run-time: RUN
1874                   lexical variables are garbage collected before END: RUN
1875                   end-time: END
1876                   package variables are garbage collected after END: DESTRUCT
1877
1878               This variable was added in Perl 5.14.0.
1879
1880       $^H     WARNING: This variable is strictly for internal use only.  Its
1881               availability, behavior, and contents are subject to change
1882               without notice.
1883
1884               This variable contains compile-time hints for the Perl
1885               interpreter.  At the end of compilation of a BLOCK the value of
1886               this variable is restored to the value when the interpreter
1887               started to compile the BLOCK.
1888
1889               When perl begins to parse any block construct that provides a
1890               lexical scope (e.g., eval body, required file, subroutine body,
1891               loop body, or conditional block), the existing value of $^H is
1892               saved, but its value is left unchanged.  When the compilation
1893               of the block is completed, it regains the saved value.  Between
1894               the points where its value is saved and restored, code that
1895               executes within BEGIN blocks is free to change the value of
1896               $^H.
1897
1898               This behavior provides the semantic of lexical scoping, and is
1899               used in, for instance, the "use strict" pragma.
1900
1901               The contents should be an integer; different bits of it are
1902               used for different pragmatic flags.  Here's an example:
1903
1904                   sub add_100 { $^H |= 0x100 }
1905
1906                   sub foo {
1907                       BEGIN { add_100() }
1908                       bar->baz($boon);
1909                   }
1910
1911               Consider what happens during execution of the BEGIN block.  At
1912               this point the BEGIN block has already been compiled, but the
1913               body of "foo()" is still being compiled.  The new value of $^H
1914               will therefore be visible only while the body of "foo()" is
1915               being compiled.
1916
1917               Substitution of "BEGIN { add_100() }" block with:
1918
1919                   BEGIN { require strict; strict->import('vars') }
1920
1921               demonstrates how "use strict 'vars'" is implemented.  Here's a
1922               conditional version of the same lexical pragma:
1923
1924                   BEGIN {
1925                       require strict; strict->import('vars') if $condition
1926                   }
1927
1928               This variable was added in Perl 5.003.
1929
1930       %^H     The "%^H" hash provides the same scoping semantic as $^H.  This
1931               makes it useful for implementation of lexically scoped pragmas.
1932               See perlpragma.   All the entries are stringified when accessed
1933               at runtime, so only simple values can be accommodated.  This
1934               means no pointers to objects, for example.
1935
1936               When putting items into "%^H", in order to avoid conflicting
1937               with other users of the hash there is a convention regarding
1938               which keys to use.  A module should use only keys that begin
1939               with the module's name (the name of its main package) and a "/"
1940               character.  For example, a module "Foo::Bar" should use keys
1941               such as "Foo::Bar/baz".
1942
1943               This variable was added in Perl v5.6.0.
1944
1945       ${^OPEN}
1946               An internal variable used by PerlIO.  A string in two parts,
1947               separated by a "\0" byte, the first part describes the input
1948               layers, the second part describes the output layers.
1949
1950               This is the mechanism that applies the lexical effects of the
1951               open pragma, and the main program scope effects of the "io" or
1952               "D" options for the -C command-line switch and PERL_UNICODE
1953               environment variable.
1954
1955               The functions "accept()", "open()", "pipe()", "readpipe()" (as
1956               well as the related "qx" and "`STRING`" operators), "socket()",
1957               "socketpair()", and "sysopen()" are affected by the lexical
1958               value of this variable.  The implicit "ARGV" handle opened by
1959               "readline()" (or the related "<>" and "<<>>" operators) on
1960               passed filenames is also affected (but not if it opens
1961               "STDIN").  If this variable is not set, these functions will
1962               set the default layers as described in "Defaults and how to
1963               override them" in PerlIO.
1964
1965               "open()" ignores this variable (and the default layers) when
1966               called with 3 arguments and explicit layers are specified.
1967               Indirect calls to these functions via modules like IO::Handle
1968               are not affected as they occur in a different lexical scope.
1969               Directory handles such as opened by "opendir()" are not
1970               currently affected.
1971
1972               This variable was added in Perl v5.8.0.
1973
1974       $PERLDB
1975       $^P     The internal variable for debugging support.  The meanings of
1976               the various bits are subject to change, but currently indicate:
1977
1978               0x01  Debug subroutine enter/exit.
1979
1980               0x02  Line-by-line debugging.  Causes "DB::DB()" subroutine to
1981                     be called for each statement executed.  Also causes
1982                     saving source code lines (like 0x400).
1983
1984               0x04  Switch off optimizations.
1985
1986               0x08  Preserve more data for future interactive inspections.
1987
1988               0x10  Keep info about source lines on which a subroutine is
1989                     defined.
1990
1991               0x20  Start with single-step on.
1992
1993               0x40  Use subroutine address instead of name when reporting.
1994
1995               0x80  Report "goto &subroutine" as well.
1996
1997               0x100 Provide informative "file" names for evals based on the
1998                     place they were compiled.
1999
2000               0x200 Provide informative names to anonymous subroutines based
2001                     on the place they were compiled.
2002
2003               0x400 Save source code lines into "@{"_<$filename"}".
2004
2005               0x800 When saving source, include evals that generate no
2006                     subroutines.
2007
2008               0x1000
2009                     When saving source, include source that did not compile.
2010
2011               Some bits may be relevant at compile-time only, some at run-
2012               time only.  This is a new mechanism and the details may change.
2013               See also perldebguts.
2014
2015       ${^TAINT}
2016               Reflects if taint mode is on or off.  1 for on (the program was
2017               run with -T), 0 for off, -1 when only taint warnings are
2018               enabled (i.e. with -t or -TU).
2019
2020               This variable is read-only.
2021
2022               This variable was added in Perl v5.8.0.
2023
2024       ${^SAFE_LOCALES}
2025               Reflects if safe locale operations are available to this perl
2026               (when the value is 1) or not (the value is 0).  This variable
2027               is always 1 if the perl has been compiled without threads.  It
2028               is also 1 if this perl is using thread-safe locale operations.
2029               Note that an individual thread may choose to use the global
2030               locale (generally unsafe) by calling "switch_to_global_locale"
2031               in perlapi.  This variable currently is still set to 1 in such
2032               threads.
2033
2034               This variable is read-only.
2035
2036               This variable was added in Perl v5.28.0.
2037
2038       ${^UNICODE}
2039               Reflects certain Unicode settings of Perl.  See perlrun
2040               documentation for the "-C" switch for more information about
2041               the possible values.
2042
2043               This variable is set during Perl startup and is thereafter
2044               read-only.
2045
2046               This variable was added in Perl v5.8.2.
2047
2048       ${^UTF8CACHE}
2049               This variable controls the state of the internal UTF-8 offset
2050               caching code.  1 for on (the default), 0 for off, -1 to debug
2051               the caching code by checking all its results against linear
2052               scans, and panicking on any discrepancy.
2053
2054               This variable was added in Perl v5.8.9.  It is subject to
2055               change or removal without notice, but is currently used to
2056               avoid recalculating the boundaries of multi-byte UTF-8-encoded
2057               characters.
2058
2059       ${^UTF8LOCALE}
2060               This variable indicates whether a UTF-8 locale was detected by
2061               perl at startup.  This information is used by perl when it's in
2062               adjust-utf8ness-to-locale mode (as when run with the "-CL"
2063               command-line switch); see perlrun for more info on this.
2064
2065               This variable was added in Perl v5.8.8.
2066
2067   Deprecated and removed variables
2068       Deprecating a variable announces the intent of the perl maintainers to
2069       eventually remove the variable from the language.  It may still be
2070       available despite its status.  Using a deprecated variable triggers a
2071       warning.
2072
2073       Once a variable is removed, its use triggers an error telling you the
2074       variable is unsupported.
2075
2076       See perldiag for details about error messages.
2077
2078       $#      $# was a variable that could be used to format printed numbers.
2079               After a deprecation cycle, its magic was removed in Perl
2080               v5.10.0 and using it now triggers a warning: "$# is no longer
2081               supported".
2082
2083               This is not the sigil you use in front of an array name to get
2084               the last index, like $#array.  That's still how you get the
2085               last index of an array in Perl.  The two have nothing to do
2086               with each other.
2087
2088               Deprecated in Perl 5.
2089
2090               Removed in Perl v5.10.0.
2091
2092       $*      $* was a variable that you could use to enable multiline
2093               matching.  After a deprecation cycle, its magic was removed in
2094               Perl v5.10.0.  Using it now triggers a warning: "$* is no
2095               longer supported".  You should use the "/s" and "/m" regexp
2096               modifiers instead.
2097
2098               Deprecated in Perl 5.
2099
2100               Removed in Perl v5.10.0.
2101
2102       $[      This variable stores the index of the first element in an
2103               array, and of the first character in a substring.  The default
2104               is 0, but you could theoretically set it to 1 to make Perl
2105               behave more like awk (or Fortran) when subscripting and when
2106               evaluating the index() and substr() functions.
2107
2108               As of release 5 of Perl, assignment to $[ is treated as a
2109               compiler directive, and cannot influence the behavior of any
2110               other file.  (That's why you can only assign compile-time
2111               constants to it.)  Its use is highly discouraged.
2112
2113               Prior to Perl v5.10.0, assignment to $[ could be seen from
2114               outer lexical scopes in the same file, unlike other compile-
2115               time directives (such as strict).  Using local() on it would
2116               bind its value strictly to a lexical block.  Now it is always
2117               lexically scoped.
2118
2119               As of Perl v5.16.0, it is implemented by the arybase module.
2120
2121               As of Perl v5.30.0, or under "use v5.16", or "no feature
2122               "array_base"", $[ no longer has any effect, and always contains
2123               0.  Assigning 0 to it is permitted, but any other value will
2124               produce an error.
2125
2126               Mnemonic: [ begins subscripts.
2127
2128               Deprecated in Perl v5.12.0.
2129
2130
2131
2132perl v5.34.1                      2022-03-15                        PERLVAR(1)
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