1PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
2
6 perlvar - Perl predefined variables
7
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 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 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. $^_
35 itself, however, is reserved.
36 Perl identifiers that begin with digits or punctuation characters are
38 exempt from the effects of the "package" declaration and are always
39 forced to be in package "main"; they are also exempt from "strict
40 'vars'" errors. A few other names are also exempt in these ways:
41 ENV STDIN
43 INC STDOUT
44 ARGV STDERR
45 ARGVOUT
46 SIG
47 In particular, the special "${^_XYZ}" variables are always taken to be
49 in package "main", regardless of any "package" declarations presently
50 in scope.
51
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 use English;
59 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 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 General Variables
72 $ARG
73 $_ The default input and pattern-searching space. The following
74 pairs are equivalent:
75 while (<>) {...} # equivalent only in while!
77 while (defined($_ = <>)) {...}
78 /^Subject:/
80 $_ =~ /^Subject:/
81 tr/a-z/A-Z/
83 $_ =~ tr/a-z/A-Z/
84 chomp
86 chomp($_)
87 Here are the places where Perl will assume $_ even if you don't
89 use it:
90 · The following functions use $_ as a default argument:
92 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 · All file tests ("-f", "-d") except for "-t", which defaults
101 to STDIN. See "-X" in perlfunc
102 · The pattern matching operations "m//", "s///" and "tr///"
104 (aka "y///") when used without an "=~" operator.
105 · The default iterator variable in a "foreach" loop if no
107 other variable is supplied.
108 · The implicit iterator variable in the "grep()" and "map()"
110 functions.
111 · The implicit variable of "given()".
113 · 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 $_ is a global variable.
120 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 Mnemonic: underline is understood in certain operations.
128 @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 See perlsub.
135 $LIST_SEPARATOR
137 $" When an array or an array slice is interpolated into a double-
138 quoted string or a similar context such as "/.../", its
139 elements are separated by this value. Default is a space. For
140 example, this:
141 print "The array is: @array\n";
143 is equivalent to this:
145 print "The array is: " . join($", @array) . "\n";
147 Mnemonic: works in double-quoted context.
149 $PROCESS_ID
151 $PID
152 $$ The process number of the Perl running this script. Though you
153 can set this variable, doing so is generally discouraged,
154 although it can be invaluable for some testing purposes. It
155 will be reset automatically across "fork()" calls.
156 Note for Linux and Debian GNU/kFreeBSD users: Before Perl
158 v5.16.0 perl would emulate POSIX semantics on Linux systems
159 using LinuxThreads, a partial implementation of POSIX Threads
160 that has since been superseded by the Native POSIX Thread
161 Library (NPTL).
162 LinuxThreads is now obsolete on Linux, and caching "getpid()"
164 like this made embedding perl unnecessarily complex (since
165 you'd have to manually update the value of $$), so now $$ and
166 "getppid()" will always return the same values as the
167 underlying C library.
168 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 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 Mnemonic: same as shells.
178 $PROGRAM_NAME
180 $0 Contains the name of the program being executed.
181 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 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 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 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 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 If the program has been given to perl via the switches "-e" or
213 "-E", $0 will contain the string "-e".
214 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 Mnemonic: same as sh and ksh.
224 $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 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 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 Mnemonic: parentheses are used to group things. The real gid
246 is the group you left, if you're running setgid.
247 $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 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 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 $<, $>, $( 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 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 $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 Mnemonic: it's the uid you came from, if you're running setuid.
285 $EFFECTIVE_USER_ID
287 $EUID
288 $> The effective uid of this process. For example:
289 $< = $>; # set real to effective uid
291 ($<,$>) = ($>,$<); # swap real and effective uids
292 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 $< and $> can be swapped only on machines supporting
299 "setreuid()".
300 Mnemonic: it's the uid you went to, if you're running setuid.
302 $SUBSCRIPT_SEPARATOR
304 $SUBSEP
305 $; The subscript separator for multidimensional array emulation.
306 If you refer to a hash element as
307 $foo{$x,$y,$z}
309 it really means
311 $foo{join($;, $x, $y, $z)}
313 But don't put
315 @foo{$x,$y,$z} # a slice--note the @
317 which means
319 ($foo{$x},$foo{$y},$foo{$z})
321 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 Consider using "real" multidimensional arrays as described in
326 perllol.
327 Mnemonic: comma (the syntactic subscript separator) is a semi-
329 semicolon.
330 $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 %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 As of v5.18.0, both keys and values stored in %ENV are
344 stringified.
345 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 Previously, only child processes received stringified values:
355 my $foo = 1;
357 $ENV{'bar'} = \$foo;
358 # Always printed 'non ref'
360 system($^X, '-e',
361 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
362 This happens because you can't really share arbitrary data
364 structures with foreign processes.
365 $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 This variable can be used to determine whether the Perl
373 interpreter executing a script is in the right range of
374 versions:
375 warn "No PerlIO!\n" if $] lt '5.008';
377 When comparing $], string comparison operators are highly
379 recommended. The inherent limitations of binary floating point
380 representation can sometimes lead to incorrect comparisons for
381 some numbers on some architectures.
382 See also the documentation of "use VERSION" and "require
384 VERSION" for a convenient way to fail if the running Perl
385 interpreter is too old.
386 See "$^V" for a representation of the Perl version as a version
388 object, which allows more flexible string comparisons.
389 The main advantage of $] over $^V is that it works the same on
391 any version of Perl. The disadvantages are that it can't
392 easily be compared to versions in other formats (e.g. literal
393 v-strings, "v1.2.3" or version objects) and numeric comparisons
394 can occasionally fail; it's good for string literal version
395 checks and bad for comparing to a variable that hasn't been
396 sanity-checked.
397 The $OLD_PERL_VERSION form was added in Perl v5.20.0 for
399 historical reasons but its use is discouraged. (If your reason
400 to use $] is to run code on old perls then referring to it as
401 $OLD_PERL_VERSION would be self-defeating.)
402 Mnemonic: Is this version of perl in the right bracket?
404 $SYSTEM_FD_MAX
406 $^F The maximum system file descriptor, ordinarily 2. System file
407 descriptors are passed to "exec()"ed processes, while higher
408 file descriptors are not. Also, during an "open()", system
409 file descriptors are preserved even if the "open()" fails
410 (ordinary file descriptors are closed before the "open()" is
411 attempted). The close-on-exec status of a file descriptor will
412 be decided according to the value of $^F when the corresponding
413 file, pipe, or socket was opened, not the time of the "exec()".
414 @F The array @F contains the fields of each line read in when
416 autosplit mode is turned on. See perlrun for the -a switch.
417 This array is package-specific, and must be declared or given a
418 full package name if not in package main when running under
419 "strict 'vars'".
420 @INC The array @INC contains the list of places that the "do EXPR",
422 "require", or "use" constructs look for their library files.
423 It initially consists of the arguments to any -I command-line
424 switches, followed by the default Perl library, probably
425 /usr/local/lib/perl. Prior to Perl 5.26, "." -which represents
426 the current directory, was included in @INC; it has been
427 removed. This change in behavior is documented in
428 "PERL_USE_UNSAFE_INC" and it is not recommended that "." be re-
429 added to @INC. If you need to modify @INC at runtime, you
430 should use the "use lib" pragma to get the machine-dependent
431 library properly loaded as well:
432 use lib '/mypath/libdir/';
434 use SomeMod;
435 You can also insert hooks into the file inclusion system by
437 putting Perl code directly into @INC. Those hooks may be
438 subroutine references, array references or blessed objects.
439 See "require" in perlfunc for details.
440 %INC The hash %INC contains entries for each filename included via
442 the "do", "require", or "use" operators. The key is the
443 filename you specified (with module names converted to
444 pathnames), and the value is the location of the file found.
445 The "require" operator uses this hash to determine whether a
446 particular file has already been included.
447 If the file was loaded via a hook (e.g. a subroutine reference,
449 see "require" in perlfunc for a description of these hooks),
450 this hook is by default inserted into %INC in place of a
451 filename. Note, however, that the hook may have set the %INC
452 entry by itself to provide some more specific info.
453 $INPLACE_EDIT
455 $^I The current value of the inplace-edit extension. Use "undef"
456 to disable inplace editing.
457 Mnemonic: value of -i switch.
459 @ISA Each package contains a special array called @ISA which
461 contains a list of that class's parent classes, if any. This
462 array is simply a list of scalars, each of which is a string
463 that corresponds to a package name. The array is examined when
464 Perl does method resolution, which is covered in perlobj.
465 To load packages while adding them to @ISA, see the parent
467 pragma. The discouraged base pragma does this as well, but
468 should not be used except when compatibility with the
469 discouraged fields pragma is required.
470 $^M By default, running out of memory is an untrappable, fatal
472 error. However, if suitably built, Perl can use the contents
473 of $^M as an emergency memory pool after "die()"ing. Suppose
474 that your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and
475 used Perl's malloc. Then
476 $^M = 'a' x (1 << 16);
478 would allocate a 64K buffer for use in an emergency. See the
480 INSTALL file in the Perl distribution for information on how to
481 add custom C compilation flags when compiling perl. To
482 discourage casual use of this advanced feature, there is no
483 English long name for this variable.
484 This variable was added in Perl 5.004.
486 $OSNAME
488 $^O The name of the operating system under which this copy of Perl
489 was built, as determined during the configuration process. For
490 examples see "PLATFORMS" in perlport.
491 The value is identical to $Config{'osname'}. See also Config
493 and the -V command-line switch documented in perlrun.
494 In Windows platforms, $^O is not very helpful: since it is
496 always "MSWin32", it doesn't tell the difference between
497 95/98/ME/NT/2000/XP/CE/.NET. Use "Win32::GetOSName()" or
498 Win32::GetOSVersion() (see Win32 and perlport) to distinguish
499 between the variants.
500 This variable was added in Perl 5.003.
502 %SIG The hash %SIG contains signal handlers for signals. For
504 example:
505 sub handler { # 1st argument is signal name
507 my($sig) = @_;
508 print "Caught a SIG$sig--shutting down\n";
509 close(LOG);
510 exit(0);
511 }
512 $SIG{'INT'} = \&handler;
514 $SIG{'QUIT'} = \&handler;
515 ...
516 $SIG{'INT'} = 'DEFAULT'; # restore default action
517 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
518 Using a value of 'IGNORE' usually has the effect of ignoring
520 the signal, except for the "CHLD" signal. See perlipc for more
521 about this special case.
522 Here are some other examples:
524 $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 Be sure not to use a bareword as the name of a signal handler,
534 lest you inadvertently call it.
535 If your system has the "sigaction()" function then signal
537 handlers are installed using it. This means you get reliable
538 signal handling.
539 The default delivery policy of signals changed in Perl v5.8.0
541 from immediate (also known as "unsafe") to deferred, also known
542 as "safe signals". See perlipc for more information.
543 Certain internal hooks can be also set using the %SIG hash.
545 The routine indicated by $SIG{__WARN__} is called when a
546 warning message is about to be printed. The warning message is
547 passed as the first argument. The presence of a "__WARN__"
548 hook causes the ordinary printing of warnings to "STDERR" to be
549 suppressed. You can use this to save warnings in a variable,
550 or turn warnings into fatal errors, like this:
551 local $SIG{__WARN__} = sub { die $_[0] };
553 eval $proggie;
554 As the 'IGNORE' hook is not supported by "__WARN__", you can
556 disable warnings using the empty subroutine:
557 local $SIG{__WARN__} = sub {};
559 The routine indicated by $SIG{__DIE__} is called when a fatal
561 exception is about to be thrown. The error message is passed
562 as the first argument. When a "__DIE__" hook routine returns,
563 the exception processing continues as it would have in the
564 absence of the hook, unless the hook routine itself exits via a
565 "goto &sub", a loop exit, or a "die()". The "__DIE__" handler
566 is explicitly disabled during the call, so that you can die
567 from a "__DIE__" handler. Similarly for "__WARN__".
568 The $SIG{__DIE__} hook is called even inside an "eval()". It
570 was never intended to happen this way, but an implementation
571 glitch made this possible. This used to be deprecated, as it
572 allowed strange action at a distance like rewriting a pending
573 exception in $@. Plans to rectify this have been scrapped, as
574 users found that rewriting a pending exception is actually a
575 useful feature, and not a bug.
576 "__DIE__"/"__WARN__" handlers are very special in one respect:
578 they may be called to report (probable) errors found by the
579 parser. In such a case the parser may be in inconsistent
580 state, so any attempt to evaluate Perl code from such a handler
581 will probably result in a segfault. This means that warnings
582 or errors that result from parsing Perl should be used with
583 extreme caution, like this:
584 require Carp if defined $^S;
586 Carp::confess("Something wrong") if defined &Carp::confess;
587 die "Something wrong, but could not load Carp to give "
588 . "backtrace...\n\t"
589 . "To see backtrace try starting Perl with -MCarp switch";
590 Here the first line will load "Carp" unless it is the parser
592 who called the handler. The second line will print backtrace
593 and die if "Carp" was available. The third line will be
594 executed only if "Carp" was not available.
595 Having to even think about the $^S variable in your exception
597 handlers is simply wrong. $SIG{__DIE__} as currently
598 implemented invites grievous and difficult to track down
599 errors. Avoid it and use an "END{}" or CORE::GLOBAL::die
600 override instead.
601 See "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc,
603 and warnings for additional information.
604 $BASETIME
606 $^T The time at which the program began running, in seconds since
607 the epoch (beginning of 1970). The values returned by the -M,
608 -A, and -C filetests are based on this value.
609 $PERL_VERSION
611 $^V The revision, version, and subversion of the Perl interpreter,
612 represented as a version object.
613 This variable first appeared in perl v5.6.0; earlier versions
615 of perl will see an undefined value. Before perl v5.10.0 $^V
616 was represented as a v-string rather than a version object.
617 $^V can be used to determine whether the Perl interpreter
619 executing a script is in the right range of versions. For
620 example:
621 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
623 While version objects overload stringification, to portably
625 convert $^V into its string representation, use "sprintf()"'s
626 "%vd" conversion, which works for both v-strings or version
627 objects:
628 printf "version is v%vd\n", $^V; # Perl's version
630 See the documentation of "use VERSION" and "require VERSION"
632 for a convenient way to fail if the running Perl interpreter is
633 too old.
634 See also "$]" for a decimal representation of the Perl version.
636 The main advantage of $^V over $] is that, for Perl v5.10.0 or
638 later, it overloads operators, allowing easy comparison against
639 other version representations (e.g. decimal, literal v-string,
640 "v1.2.3", or objects). The disadvantage is that prior to
641 v5.10.0, it was only a literal v-string, which can't be easily
642 printed or compared, whereas the behavior of $] is unchanged on
643 all versions of Perl.
644 Mnemonic: use ^V for a version object.
646 ${^WIN32_SLOPPY_STAT}
648 If this variable is set to a true value, then "stat()" on
649 Windows will not try to open the file. This means that the
650 link count cannot be determined and file attributes may be out
651 of date if additional hardlinks to the file exist. On the
652 other hand, not opening the file is considerably faster,
653 especially for files on network drives.
654 This variable could be set in the sitecustomize.pl file to
656 configure the local Perl installation to use "sloppy" "stat()"
657 by default. See the documentation for -f in perlrun for more
658 information about site customization.
659 This variable was added in Perl v5.10.0.
661 $EXECUTABLE_NAME
663 $^X The name used to execute the current copy of Perl, from C's
664 "argv[0]" or (where supported) /proc/self/exe.
665 Depending on the host operating system, the value of $^X may be
667 a relative or absolute pathname of the perl program file, or
668 may be the string used to invoke perl but not the pathname of
669 the perl program file. Also, most operating systems permit
670 invoking programs that are not in the PATH environment
671 variable, so there is no guarantee that the value of $^X is in
672 PATH. For VMS, the value may or may not include a version
673 number.
674 You usually can use the value of $^X to re-invoke an
676 independent copy of the same perl that is currently running,
677 e.g.,
678 @first_run = `$^X -le "print int rand 100 for 1..100"`;
680 But recall that not all operating systems support forking or
682 capturing of the output of commands, so this complex statement
683 may not be portable.
684 It is not safe to use the value of $^X as a path name of a
686 file, as some operating systems that have a mandatory suffix on
687 executable files do not require use of the suffix when invoking
688 a command. To convert the value of $^X to a path name, use the
689 following statements:
690 # Build up a set of file names (not command names).
692 use Config;
693 my $this_perl = $^X;
694 if ($^O ne 'VMS') {
695 $this_perl .= $Config{_exe}
696 unless $this_perl =~ m/$Config{_exe}$/i;
697 }
698 Because many operating systems permit anyone with read access
700 to the Perl program file to make a copy of it, patch the copy,
701 and then execute the copy, the security-conscious Perl
702 programmer should take care to invoke the installed copy of
703 perl, not the copy referenced by $^X. The following statements
704 accomplish this goal, and produce a pathname that can be
705 invoked as a command or referenced as a file.
706 use Config;
708 my $secure_perl_path = $Config{perlpath};
709 if ($^O ne 'VMS') {
710 $secure_perl_path .= $Config{_exe}
711 unless $secure_perl_path =~ m/$Config{_exe}$/i;
712 }
713 Variables related to regular expressions
715 Most of the special variables related to regular expressions are side
716 effects. Perl sets these variables when it has a successful match, so
717 you should check the match result before using them. For instance:
718 if( /P(A)TT(ER)N/ ) {
720 print "I found $1 and $2\n";
721 }
722 These variables are read-only and dynamically-scoped, unless we note
724 otherwise.
725 The dynamic nature of the regular expression variables means that their
727 value is limited to the block that they are in, as demonstrated by this
728 bit of code:
729 my $outer = 'Wallace and Grommit';
731 my $inner = 'Mutt and Jeff';
732 my $pattern = qr/(\S+) and (\S+)/;
734 sub show_n { print "\$1 is $1; \$2 is $2\n" }
736 {
738 OUTER:
739 show_n() if $outer =~ m/$pattern/;
740 INNER: {
742 show_n() if $inner =~ m/$pattern/;
743 }
744 show_n();
746 }
747 The output shows that while in the "OUTER" block, the values of $1 and
749 $2 are from the match against $outer. Inside the "INNER" block, the
750 values of $1 and $2 are from the match against $inner, but only until
751 the end of the block (i.e. the dynamic scope). After the "INNER" block
752 completes, the values of $1 and $2 return to the values for the match
753 against $outer even though we have not made another match:
754 $1 is Wallace; $2 is Grommit
756 $1 is Mutt; $2 is Jeff
757 $1 is Wallace; $2 is Grommit
758 Performance issues
760 Traditionally in Perl, any use of any of the three variables "$`", $&
762 or "$'" (or their "use English" equivalents) anywhere in the code,
763 caused all subsequent successful pattern matches to make a copy of the
764 matched string, in case the code might subsequently access one of those
765 variables. This imposed a considerable performance penalty across the
766 whole program, so generally the use of these variables has been
767 discouraged.
768 In Perl 5.6.0 the "@-" and "@+" dynamic arrays were introduced that
770 supply the indices of successful matches. So you could for example do
771 this:
772 $str =~ /pattern/;
774 print $`, $&, $'; # bad: performance hit
776 print # good: no performance hit
778 substr($str, 0, $-[0]),
779 substr($str, $-[0], $+[0]-$-[0]),
780 substr($str, $+[0]);
781 In Perl 5.10.0 the "/p" match operator flag and the "${^PREMATCH}",
783 "${^MATCH}", and "${^POSTMATCH}" variables were introduced, that
784 allowed you to suffer the penalties only on patterns marked with "/p".
785 In Perl 5.18.0 onwards, perl started noting the presence of each of the
787 three variables separately, and only copied that part of the string
788 required; so in
789 $`; $&; "abcdefgh" =~ /d/
791 perl would only copy the "abcd" part of the string. That could make a
793 big difference in something like
794 $str = 'x' x 1_000_000;
796 $&; # whoops
797 $str =~ /x/g # one char copied a million times, not a million chars
798 In Perl 5.20.0 a new copy-on-write system was enabled by default, which
800 finally fixes all performance issues with these three variables, and
801 makes them safe to use anywhere.
802 The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you
804 find uses of these problematic match variables in your code.
805 $<digits> ($1, $2, ...)
807 Contains the subpattern from the corresponding set of capturing
808 parentheses from the last successful pattern match, not
809 counting patterns matched in nested blocks that have been
810 exited already.
811 Note there is a distinction between a capture buffer which
813 matches the empty string a capture buffer which is optional.
814 Eg, "(x?)" and "(x)?" The latter may be undef, the former not.
815 These variables are read-only and dynamically-scoped.
817 Mnemonic: like \digits.
819 @{^CAPTURE}
821 An array which exposes the contents of the capture buffers, if
822 any, of the last successful pattern match, not counting
823 patterns matched in nested blocks that have been exited
824 already.
825 Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1
827 index is equivalent to $2, etc.
828 if ("foal"=~/(.)(.)(.)(.)/) {
830 print join "-", @{^CAPTURE};
831 }
832 should output "f-o-a-l".
834 See also "$<digits> ($1, $2, ...)", "%{^CAPTURE}" and
836 "%{^CAPTURE_ALL}".
837 Note that unlike most other regex magic variables there is no
839 single letter equivalent to "@{^CAPTURE}".
840 This variable was added in 5.25.7
842 $MATCH
844 $& The string matched by the last successful pattern match (not
845 counting any matches hidden within a BLOCK or "eval()" enclosed
846 by the current BLOCK).
847 See "Performance issues" above for the serious performance
849 implications of using this variable (even once) in your code.
850 This variable is read-only and dynamically-scoped.
852 Mnemonic: like "&" in some editors.
854 ${^MATCH}
856 This is similar to $& ($MATCH) except that it does not incur
857 the performance penalty associated with that variable.
858 See "Performance issues" above.
860 In Perl v5.18 and earlier, it is only guaranteed to return a
862 defined value when the pattern was compiled or executed with
863 the "/p" modifier. In Perl v5.20, the "/p" modifier does
864 nothing, so "${^MATCH}" does the same thing as $MATCH.
865 This variable was added in Perl v5.10.0.
867 This variable is read-only and dynamically-scoped.
869 $PREMATCH
871 $` The string preceding whatever was matched by the last
872 successful pattern match, not counting any matches hidden
873 within a BLOCK or "eval" enclosed by the current BLOCK.
874 See "Performance issues" above for the serious performance
876 implications of using this variable (even once) in your code.
877 This variable is read-only and dynamically-scoped.
879 Mnemonic: "`" often precedes a quoted string.
881 ${^PREMATCH}
883 This is similar to "$`" ($PREMATCH) except that it does not
884 incur the performance penalty associated with that variable.
885 See "Performance issues" above.
887 In Perl v5.18 and earlier, it is only guaranteed to return a
889 defined value when the pattern was compiled or executed with
890 the "/p" modifier. In Perl v5.20, the "/p" modifier does
891 nothing, so "${^PREMATCH}" does the same thing as $PREMATCH.
892 This variable was added in Perl v5.10.0.
894 This variable is read-only and dynamically-scoped.
896 $POSTMATCH
898 $' The string following whatever was matched by the last
899 successful pattern match (not counting any matches hidden
900 within a BLOCK or "eval()" enclosed by the current BLOCK).
901 Example:
902 local $_ = 'abcdefghi';
904 /def/;
905 print "$`:$&:$'\n"; # prints abc:def:ghi
906 See "Performance issues" above for the serious performance
908 implications of using this variable (even once) in your code.
909 This variable is read-only and dynamically-scoped.
911 Mnemonic: "'" often follows a quoted string.
913 ${^POSTMATCH}
915 This is similar to "$'" ($POSTMATCH) except that it does not
916 incur the performance penalty associated with that variable.
917 See "Performance issues" above.
919 In Perl v5.18 and earlier, it is only guaranteed to return a
921 defined value when the pattern was compiled or executed with
922 the "/p" modifier. In Perl v5.20, the "/p" modifier does
923 nothing, so "${^POSTMATCH}" does the same thing as $POSTMATCH.
924 This variable was added in Perl v5.10.0.
926 This variable is read-only and dynamically-scoped.
928 $LAST_PAREN_MATCH
930 $+ The text matched by the highest used capture group of the last
931 successful search pattern. It is logically equivalent to the
932 highest numbered capture variable ($1, $2, ...) which has a
933 defined value.
934 This is useful if you don't know which one of a set of
936 alternative patterns matched. For example:
937 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
939 This variable is read-only and dynamically-scoped.
941 Mnemonic: be positive and forward looking.
943 $LAST_SUBMATCH_RESULT
945 $^N The text matched by the used group most-recently closed (i.e.
946 the group with the rightmost closing parenthesis) of the last
947 successful search pattern. This is subtly different from $+.
948 For example in
949 "ab" =~ /^((.)(.))$/
951 we have
953 $1,$^N have the value "ab"
955 $2 has the value "a"
956 $3,$+ have the value "b"
957 This is primarily used inside "(?{...})" blocks for examining
959 text recently matched. For example, to effectively capture
960 text to a variable (in addition to $1, $2, etc.), replace
961 "(...)" with
962 (?:(...)(?{ $var = $^N }))
964 By setting and then using $var in this way relieves you from
966 having to worry about exactly which numbered set of parentheses
967 they are.
968 This variable was added in Perl v5.8.0.
970 Mnemonic: the (possibly) Nested parenthesis that most recently
972 closed.
973 @LAST_MATCH_END
975 @+ This array holds the offsets of the ends of the last successful
976 submatches in the currently active dynamic scope. $+[0] is the
977 offset into the string of the end of the entire match. This is
978 the same value as what the "pos" function returns when called
979 on the variable that was matched against. The nth element of
980 this array holds the offset of the nth submatch, so $+[1] is
981 the offset past where $1 ends, $+[2] the offset past where $2
982 ends, and so on. You can use $#+ to determine how many
983 subgroups were in the last successful match. See the examples
984 given for the "@-" variable.
985 This variable was added in Perl v5.6.0.
987 %{^CAPTURE}
989 %LAST_PAREN_MATCH
990 %+ Similar to "@+", the "%+" hash allows access to the named
991 capture buffers, should they exist, in the last successful
992 match in the currently active dynamic scope.
993 For example, $+{foo} is equivalent to $1 after the following
995 match:
996 'foo' =~ /(?<foo>foo)/;
998 The keys of the "%+" hash list only the names of buffers that
1000 have captured (and that are thus associated to defined values).
1001 If multiple distinct capture groups have the same name, then
1003 $+{NAME} will refer to the leftmost defined group in the match.
1004 The underlying behaviour of "%+" is provided by the
1006 Tie::Hash::NamedCapture module.
1007 Note: "%-" and "%+" are tied views into a common internal hash
1009 associated with the last successful regular expression.
1010 Therefore mixing iterative access to them via "each" may have
1011 unpredictable results. Likewise, if the last successful match
1012 changes, then the results may be surprising.
1013 This variable was added in Perl v5.10.0. The "%{^CAPTURE}"
1015 alias was added in 5.25.7.
1016 This variable is read-only and dynamically-scoped.
1018 @LAST_MATCH_START
1020 @- "$-[0]" is the offset of the start of the last successful
1021 match. "$-[n]" is the offset of the start of the substring
1022 matched by n-th subpattern, or undef if the subpattern did not
1023 match.
1024 Thus, after a match against $_, $& coincides with "substr $_,
1026 $-[0], $+[0] - $-[0]". Similarly, $n coincides with "substr
1027 $_, $-[n], $+[n] - $-[n]" if "$-[n]" is defined, and $+
1028 coincides with "substr $_, $-[$#-], $+[$#-] - $-[$#-]". One
1029 can use "$#-" to find the last matched subgroup in the last
1030 successful match. Contrast with $#+, the number of subgroups
1031 in the regular expression. Compare with "@+".
1032 This array holds the offsets of the beginnings of the last
1034 successful submatches in the currently active dynamic scope.
1035 "$-[0]" is the offset into the string of the beginning of the
1036 entire match. The nth element of this array holds the offset
1037 of the nth submatch, so "$-[1]" is the offset where $1 begins,
1038 "$-[2]" the offset where $2 begins, and so on.
1039 After a match against some variable $var:
1041 "$`" is the same as "substr($var, 0, $-[0])"
1043 $& is the same as "substr($var, $-[0], $+[0] - $-[0])"
1044 "$'" is the same as "substr($var, $+[0])"
1045 $1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
1046 $2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
1047 $3 is the same as "substr($var, $-[3], $+[3] - $-[3])"
1048 This variable was added in Perl v5.6.0.
1050 %{^CAPTURE_ALL}
1052 %- Similar to "%+", this variable allows access to the named
1053 capture groups in the last successful match in the currently
1054 active dynamic scope. To each capture group name found in the
1055 regular expression, it associates a reference to an array
1056 containing the list of values captured by all buffers with that
1057 name (should there be several of them), in the order where they
1058 appear.
1059 Here's an example:
1061 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1063 foreach my $bufname (sort keys %-) {
1064 my $ary = $-{$bufname};
1065 foreach my $idx (0..$#$ary) {
1066 print "\$-{$bufname}[$idx] : ",
1067 (defined($ary->[$idx])
1068 ? "'$ary->[$idx]'"
1069 : "undef"),
1070 "\n";
1071 }
1072 }
1073 }
1074 would print out:
1076 $-{A}[0] : '1'
1078 $-{A}[1] : '3'
1079 $-{B}[0] : '2'
1080 $-{B}[1] : '4'
1081 The keys of the "%-" hash correspond to all buffer names found
1083 in the regular expression.
1084 The behaviour of "%-" is implemented via the
1086 Tie::Hash::NamedCapture module.
1087 Note: "%-" and "%+" are tied views into a common internal hash
1089 associated with the last successful regular expression.
1090 Therefore mixing iterative access to them via "each" may have
1091 unpredictable results. Likewise, if the last successful match
1092 changes, then the results may be surprising.
1093 This variable was added in Perl v5.10.0. The "%{^CAPTURE_ALL}"
1095 alias was added in 5.25.7.
1096 This variable is read-only and dynamically-scoped.
1098 $LAST_REGEXP_CODE_RESULT
1100 $^R The result of evaluation of the last successful "(?{ code })"
1101 regular expression assertion (see perlre). May be written to.
1102 This variable was added in Perl 5.005.
1104 ${^RE_COMPILE_RECURSION_LIMIT}
1106 The current value giving the maximum number of open but
1107 unclosed parenthetical groups there may be at any point during
1108 a regular expression compilation. The default is currently
1109 1000 nested groups. You may adjust it depending on your needs
1110 and the amount of memory available.
1111 This variable was added in Perl v5.30.0.
1113 ${^RE_DEBUG_FLAGS}
1115 The current value of the regex debugging flags. Set to 0 for
1116 no debug output even when the "re 'debug'" module is loaded.
1117 See re for details.
1118 This variable was added in Perl v5.10.0.
1120 ${^RE_TRIE_MAXBUF}
1122 Controls how certain regex optimisations are applied and how
1123 much memory they utilize. This value by default is 65536 which
1124 corresponds to a 512kB temporary cache. Set this to a higher
1125 value to trade memory for speed when matching large
1126 alternations. Set it to a lower value if you want the
1127 optimisations to be as conservative of memory as possible but
1128 still occur, and set it to a negative value to prevent the
1129 optimisation and conserve the most memory. Under normal
1130 situations this variable should be of no interest to you.
1131 This variable was added in Perl v5.10.0.
1133 Variables related to filehandles
1135 Variables that depend on the currently selected filehandle may be set
1136 by calling an appropriate object method on the "IO::Handle" object,
1137 although this is less efficient than using the regular built-in
1138 variables. (Summary lines below for this contain the word HANDLE.)
1139 First you must say
1140 use IO::Handle;
1142 after which you may use either
1144 method HANDLE EXPR
1146 or more safely,
1148 HANDLE->method(EXPR)
1150 Each method returns the old value of the "IO::Handle" attribute. The
1152 methods each take an optional EXPR, which, if supplied, specifies the
1153 new value for the "IO::Handle" attribute in question. If not supplied,
1154 most methods do nothing to the current value--except for "autoflush()",
1155 which will assume a 1 for you, just to be different.
1156 Because loading in the "IO::Handle" class is an expensive operation,
1158 you should learn how to use the regular built-in variables.
1159 A few of these variables are considered "read-only". This means that
1161 if you try to assign to this variable, either directly or indirectly
1162 through a reference, you'll raise a run-time exception.
1163 You should be very careful when modifying the default values of most
1165 special variables described in this document. In most cases you want
1166 to localize these variables before changing them, since if you don't,
1167 the change may affect other modules which rely on the default values of
1168 the special variables that you have changed. This is one of the
1169 correct ways to read the whole file at once:
1170 open my $fh, "<", "foo" or die $!;
1172 local $/; # enable localized slurp mode
1173 my $content = <$fh>;
1174 close $fh;
1175 But the following code is quite bad:
1177 open my $fh, "<", "foo" or die $!;
1179 undef $/; # enable slurp mode
1180 my $content = <$fh>;
1181 close $fh;
1182 since some other module, may want to read data from some file in the
1184 default "line mode", so if the code we have just presented has been
1185 executed, the global value of $/ is now changed for any other code
1186 running inside the same Perl interpreter.
1187 Usually when a variable is localized you want to make sure that this
1189 change affects the shortest scope possible. So unless you are already
1190 inside some short "{}" block, you should create one yourself. For
1191 example:
1192 my $content = '';
1194 open my $fh, "<", "foo" or die $!;
1195 {
1196 local $/;
1197 $content = <$fh>;
1198 }
1199 close $fh;
1200 Here is an example of how your own code can go broken:
1202 for ( 1..3 ){
1204 $\ = "\r\n";
1205 nasty_break();
1206 print "$_";
1207 }
1208 sub nasty_break {
1210 $\ = "\f";
1211 # do something with $_
1212 }
1213 You probably expect this code to print the equivalent of
1215 "1\r\n2\r\n3\r\n"
1217 but instead you get:
1219 "1\f2\f3\f"
1221 Why? Because "nasty_break()" modifies "$\" without localizing it first.
1223 The value you set in "nasty_break()" is still there when you return.
1224 The fix is to add "local()" so the value doesn't leak out of
1225 "nasty_break()":
1226 local $\ = "\f";
1228 It's easy to notice the problem in such a short example, but in more
1230 complicated code you are looking for trouble if you don't localize
1231 changes to the special variables.
1232 $ARGV Contains the name of the current file when reading from "<>".
1234 @ARGV The array @ARGV contains the command-line arguments intended
1236 for the script. $#ARGV is generally the number of arguments
1237 minus one, because $ARGV[0] is the first argument, not the
1238 program's command name itself. See "$0" for the command name.
1239 ARGV The special filehandle that iterates over command-line
1241 filenames in @ARGV. Usually written as the null filehandle in
1242 the angle operator "<>". Note that currently "ARGV" only has
1243 its magical effect within the "<>" operator; elsewhere it is
1244 just a plain filehandle corresponding to the last file opened
1245 by "<>". In particular, passing "\*ARGV" as a parameter to a
1246 function that expects a filehandle may not cause your function
1247 to automatically read the contents of all the files in @ARGV.
1248 ARGVOUT The special filehandle that points to the currently open output
1250 file when doing edit-in-place processing with -i. Useful when
1251 you have to do a lot of inserting and don't want to keep
1252 modifying $_. See perlrun for the -i switch.
1253 IO::Handle->output_field_separator( EXPR )
1255 $OUTPUT_FIELD_SEPARATOR
1256 $OFS
1257 $, The output field separator for the print operator. If defined,
1258 this value is printed between each of print's arguments.
1259 Default is "undef".
1260 You cannot call "output_field_separator()" on a handle, only as
1262 a static method. See IO::Handle.
1263 Mnemonic: what is printed when there is a "," in your print
1265 statement.
1266 HANDLE->input_line_number( EXPR )
1268 $INPUT_LINE_NUMBER
1269 $NR
1270 $. Current line number for the last filehandle accessed.
1271 Each filehandle in Perl counts the number of lines that have
1273 been read from it. (Depending on the value of $/, Perl's idea
1274 of what constitutes a line may not match yours.) When a line
1275 is read from a filehandle (via "readline()" or "<>"), or when
1276 "tell()" or "seek()" is called on it, $. becomes an alias to
1277 the line counter for that filehandle.
1278 You can adjust the counter by assigning to $., but this will
1280 not actually move the seek pointer. Localizing $. will not
1281 localize the filehandle's line count. Instead, it will
1282 localize perl's notion of which filehandle $. is currently
1283 aliased to.
1284 $. is reset when the filehandle is closed, but not when an open
1286 filehandle is reopened without an intervening "close()". For
1287 more details, see "I/O Operators" in perlop. Because "<>"
1288 never does an explicit close, line numbers increase across
1289 "ARGV" files (but see examples in "eof" in perlfunc).
1290 You can also use "HANDLE->input_line_number(EXPR)" to access
1292 the line counter for a given filehandle without having to worry
1293 about which handle you last accessed.
1294 Mnemonic: many programs use "." to mean the current line
1296 number.
1297 IO::Handle->input_record_separator( EXPR )
1299 $INPUT_RECORD_SEPARATOR
1300 $RS
1301 $/ The input record separator, newline by default. This
1302 influences Perl's idea of what a "line" is. Works like awk's
1303 RS variable, including treating empty lines as a terminator if
1304 set to the null string (an empty line cannot contain any spaces
1305 or tabs). You may set it to a multi-character string to match
1306 a multi-character terminator, or to "undef" to read through the
1307 end of file. Setting it to "\n\n" means something slightly
1308 different than setting to "", if the file contains consecutive
1309 empty lines. Setting to "" will treat two or more consecutive
1310 empty lines as a single empty line. Setting to "\n\n" will
1311 blindly assume that the next input character belongs to the
1312 next paragraph, even if it's a newline.
1313 local $/; # enable "slurp" mode
1315 local $_ = <FH>; # whole file now here
1316 s/\n[ \t]+/ /g;
1317 Remember: the value of $/ is a string, not a regex. awk has to
1319 be better for something. :-)
1320 Setting $/ to an empty string -- the so-called paragraph mode
1322 -- merits special attention. When $/ is set to "" and the
1323 entire file is read in with that setting, any sequence of
1324 consecutive newlines "\n\n" at the beginning of the file is
1325 discarded. With the exception of the final record in the file,
1326 each sequence of characters ending in two or more newlines is
1327 treated as one record and is read in to end in exactly two
1328 newlines. If the last record in the file ends in zero or one
1329 consecutive newlines, that record is read in with that number
1330 of newlines. If the last record ends in two or more
1331 consecutive newlines, it is read in with two newlines like all
1332 preceding records.
1333 Suppose we wrote the following string to a file:
1335 my $string = "\n\n\n";
1337 $string .= "alpha beta\ngamma delta\n\n\n";
1338 $string .= "epsilon zeta eta\n\n";
1339 $string .= "theta\n";
1340 my $file = 'simple_file.txt';
1342 open my $OUT, '>', $file or die;
1343 print $OUT $string;
1344 close $OUT or die;
1345 Now we read that file in paragraph mode:
1347 local $/ = ""; # paragraph mode
1349 open my $IN, '<', $file or die;
1350 my @records = <$IN>;
1351 close $IN or die;
1352 @records will consist of these 3 strings:
1354 (
1356 "alpha beta\ngamma delta\n\n",
1357 "epsilon zeta eta\n\n",
1358 "theta\n",
1359 )
1360 Setting $/ to a reference to an integer, scalar containing an
1362 integer, or scalar that's convertible to an integer will
1363 attempt to read records instead of lines, with the maximum
1364 record size being the referenced integer number of characters.
1365 So this:
1366 local $/ = \32768; # or \"32768", or \$var_containing_32768
1368 open my $fh, "<", $myfile or die $!;
1369 local $_ = <$fh>;
1370 will read a record of no more than 32768 characters from $fh.
1372 If you're not reading from a record-oriented file (or your OS
1373 doesn't have record-oriented files), then you'll likely get a
1374 full chunk of data with every read. If a record is larger than
1375 the record size you've set, you'll get the record back in
1376 pieces. Trying to set the record size to zero or less is
1377 deprecated and will cause $/ to have the value of "undef",
1378 which will cause reading in the (rest of the) whole file.
1379 As of 5.19.9 setting $/ to any other form of reference will
1381 throw a fatal exception. This is in preparation for supporting
1382 new ways to set $/ in the future.
1383 On VMS only, record reads bypass PerlIO layers and any
1385 associated buffering, so you must not mix record and non-record
1386 reads on the same filehandle. Record mode mixes with line mode
1387 only when the same buffering layer is in use for both modes.
1388 You cannot call "input_record_separator()" on a handle, only as
1390 a static method. See IO::Handle.
1391 See also "Newlines" in perlport. Also see "$.".
1393 Mnemonic: / delimits line boundaries when quoting poetry.
1395 IO::Handle->output_record_separator( EXPR )
1397 $OUTPUT_RECORD_SEPARATOR
1398 $ORS
1399 $\ The output record separator for the print operator. If
1400 defined, this value is printed after the last of print's
1401 arguments. Default is "undef".
1402 You cannot call "output_record_separator()" on a handle, only
1404 as a static method. See IO::Handle.
1405 Mnemonic: you set "$\" instead of adding "\n" at the end of the
1407 print. Also, it's just like $/, but it's what you get "back"
1408 from Perl.
1409 HANDLE->autoflush( EXPR )
1411 $OUTPUT_AUTOFLUSH
1412 $| If set to nonzero, forces a flush right away and after every
1413 write or print on the currently selected output channel.
1414 Default is 0 (regardless of whether the channel is really
1415 buffered by the system or not; $| tells you only whether you've
1416 asked Perl explicitly to flush after each write). STDOUT will
1417 typically be line buffered if output is to the terminal and
1418 block buffered otherwise. Setting this variable is useful
1419 primarily when you are outputting to a pipe or socket, such as
1420 when you are running a Perl program under rsh and want to see
1421 the output as it's happening. This has no effect on input
1422 buffering. See "getc" in perlfunc for that. See "select" in
1423 perlfunc on how to select the output channel. See also
1424 IO::Handle.
1425 Mnemonic: when you want your pipes to be piping hot.
1427 ${^LAST_FH}
1429 This read-only variable contains a reference to the last-read
1430 filehandle. This is set by "<HANDLE>", "readline", "tell",
1431 "eof" and "seek". This is the same handle that $. and "tell"
1432 and "eof" without arguments use. It is also the handle used
1433 when Perl appends ", <STDIN> line 1" to an error or warning
1434 message.
1435 This variable was added in Perl v5.18.0.
1437 Variables related to formats
1439 The special variables for formats are a subset of those for
1441 filehandles. See perlform for more information about Perl's formats.
1442 $ACCUMULATOR
1444 $^A The current value of the "write()" accumulator for "format()"
1445 lines. A format contains "formline()" calls that put their
1446 result into $^A. After calling its format, "write()" prints
1447 out the contents of $^A and empties. So you never really see
1448 the contents of $^A unless you call "formline()" yourself and
1449 then look at it. See perlform and "formline PICTURE,LIST" in
1450 perlfunc.
1451 IO::Handle->format_formfeed(EXPR)
1453 $FORMAT_FORMFEED
1454 $^L What formats output as a form feed. The default is "\f".
1455 You cannot call "format_formfeed()" on a handle, only as a
1457 static method. See IO::Handle.
1458 HANDLE->format_page_number(EXPR)
1460 $FORMAT_PAGE_NUMBER
1461 $% The current page number of the currently selected output
1462 channel.
1463 Mnemonic: "%" is page number in nroff.
1465 HANDLE->format_lines_left(EXPR)
1467 $FORMAT_LINES_LEFT
1468 $- The number of lines left on the page of the currently selected
1469 output channel.
1470 Mnemonic: lines_on_page - lines_printed.
1472 IO::Handle->format_line_break_characters EXPR
1474 $FORMAT_LINE_BREAK_CHARACTERS
1475 $: The current set of characters after which a string may be
1476 broken to fill continuation fields (starting with "^") in a
1477 format. The default is " \n-", to break on a space, newline,
1478 or a hyphen.
1479 You cannot call "format_line_break_characters()" on a handle,
1481 only as a static method. See IO::Handle.
1482 Mnemonic: a "colon" in poetry is a part of a line.
1484 HANDLE->format_lines_per_page(EXPR)
1486 $FORMAT_LINES_PER_PAGE
1487 $= The current page length (printable lines) of the currently
1488 selected output channel. The default is 60.
1489 Mnemonic: = has horizontal lines.
1491 HANDLE->format_top_name(EXPR)
1493 $FORMAT_TOP_NAME
1494 $^ The name of the current top-of-page format for the currently
1495 selected output channel. The default is the name of the
1496 filehandle with "_TOP" appended. For example, the default
1497 format top name for the "STDOUT" filehandle is "STDOUT_TOP".
1498 Mnemonic: points to top of page.
1500 HANDLE->format_name(EXPR)
1502 $FORMAT_NAME
1503 $~ The name of the current report format for the currently
1504 selected output channel. The default format name is the same
1505 as the filehandle name. For example, the default format name
1506 for the "STDOUT" filehandle is just "STDOUT".
1507 Mnemonic: brother to $^.
1509 Error Variables
1511 The variables $@, $!, $^E, and $? contain information about different
1512 types of error conditions that may appear during execution of a Perl
1513 program. The variables are shown ordered by the "distance" between the
1514 subsystem which reported the error and the Perl process. They
1515 correspond to errors detected by the Perl interpreter, C library,
1516 operating system, or an external program, respectively.
1517 To illustrate the differences between these variables, consider the
1519 following Perl expression, which uses a single-quoted string. After
1520 execution of this statement, perl may have set all four special error
1521 variables:
1522 eval q{
1524 open my $pipe, "/cdrom/install |" or die $!;
1525 my @res = <$pipe>;
1526 close $pipe or die "bad pipe: $?, $!";
1527 };
1528 When perl executes the "eval()" expression, it translates the "open()",
1530 "<PIPE>", and "close" calls in the C run-time library and thence to the
1531 operating system kernel. perl sets $! to the C library's "errno" if
1532 one of these calls fails.
1533 $@ is set if the string to be "eval"-ed did not compile (this may
1535 happen if "open" or "close" were imported with bad prototypes), or if
1536 Perl code executed during evaluation "die()"d. In these cases the
1537 value of $@ is the compile error, or the argument to "die" (which will
1538 interpolate $! and $?). (See also Fatal, though.)
1539 Under a few operating systems, $^E may contain a more verbose error
1541 indicator, such as in this case, "CDROM tray not closed." Systems that
1542 do not support extended error messages leave $^E the same as $!.
1543 Finally, $? may be set to a non-0 value if the external program
1545 /cdrom/install fails. The upper eight bits reflect specific error
1546 conditions encountered by the program (the program's "exit()" value).
1547 The lower eight bits reflect mode of failure, like signal death and
1548 core dump information. See wait(2) for details. In contrast to $! and
1549 $^E, which are set only if an error condition is detected, the variable
1550 $? is set on each "wait" or pipe "close", overwriting the old value.
1551 This is more like $@, which on every "eval()" is always set on failure
1552 and cleared on success.
1553 For more details, see the individual descriptions at $@, $!, $^E, and
1555 $?.
1556 ${^CHILD_ERROR_NATIVE}
1558 The native status returned by the last pipe close, backtick
1559 ("``") command, successful call to "wait()" or "waitpid()", or
1560 from the "system()" operator. On POSIX-like systems this value
1561 can be decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED,
1562 WTERMSIG, WIFSTOPPED, WSTOPSIG and WIFCONTINUED functions
1563 provided by the POSIX module.
1564 Under VMS this reflects the actual VMS exit status; i.e. it is
1566 the same as $? when the pragma "use vmsish 'status'" is in
1567 effect.
1568 This variable was added in Perl v5.10.0.
1570 $EXTENDED_OS_ERROR
1572 $^E Error information specific to the current operating system. At
1573 the moment, this differs from "$!" under only VMS, OS/2, and
1574 Win32 (and for MacPerl). On all other platforms, $^E is always
1575 just the same as $!.
1576 Under VMS, $^E provides the VMS status value from the last
1578 system error. This is more specific information about the last
1579 system error than that provided by $!. This is particularly
1580 important when $! is set to EVMSERR.
1581 Under OS/2, $^E is set to the error code of the last call to
1583 OS/2 API either via CRT, or directly from perl.
1584 Under Win32, $^E always returns the last error information
1586 reported by the Win32 call "GetLastError()" which describes the
1587 last error from within the Win32 API. Most Win32-specific code
1588 will report errors via $^E. ANSI C and Unix-like calls set
1589 "errno" and so most portable Perl code will report errors via
1590 $!.
1591 Caveats mentioned in the description of "$!" generally apply to
1593 $^E, also.
1594 This variable was added in Perl 5.003.
1596 Mnemonic: Extra error explanation.
1598 $EXCEPTIONS_BEING_CAUGHT
1600 $^S Current state of the interpreter.
1601 $^S State
1603 --------- -------------------------------------
1604 undef Parsing module, eval, or main program
1605 true (1) Executing an eval
1606 false (0) Otherwise
1607 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
1609 handlers.
1610 The English name $EXCEPTIONS_BEING_CAUGHT is slightly
1612 misleading, because the "undef" value does not indicate whether
1613 exceptions are being caught, since compilation of the main
1614 program does not catch exceptions.
1615 This variable was added in Perl 5.004.
1617 $WARNING
1619 $^W The current value of the warning switch, initially true if -w
1620 was used, false otherwise, but directly modifiable.
1621 See also warnings.
1623 Mnemonic: related to the -w switch.
1625 ${^WARNING_BITS}
1627 The current set of warning checks enabled by the "use warnings"
1628 pragma. It has the same scoping as the $^H and "%^H"
1629 variables. The exact values are considered internal to the
1630 warnings pragma and may change between versions of Perl.
1631 This variable was added in Perl v5.6.0.
1633 $OS_ERROR
1635 $ERRNO
1636 $! When referenced, $! retrieves the current value of the C
1637 "errno" integer variable. If $! is assigned a numerical value,
1638 that value is stored in "errno". When referenced as a string,
1639 $! yields the system error string corresponding to "errno".
1640 Many system or library calls set "errno" if they fail, to
1642 indicate the cause of failure. They usually do not set "errno"
1643 to zero if they succeed. This means "errno", hence $!, is
1644 meaningful only immediately after a failure:
1645 if (open my $fh, "<", $filename) {
1647 # Here $! is meaningless.
1648 ...
1649 }
1650 else {
1651 # ONLY here is $! meaningful.
1652 ...
1653 # Already here $! might be meaningless.
1654 }
1655 # Since here we might have either success or failure,
1656 # $! is meaningless.
1657 Here, meaningless means that $! may be unrelated to the outcome
1659 of the "open()" operator. Assignment to $! is similarly
1660 ephemeral. It can be used immediately before invoking the
1661 "die()" operator, to set the exit value, or to inspect the
1662 system error string corresponding to error n, or to restore $!
1663 to a meaningful state.
1664 Mnemonic: What just went bang?
1666 %OS_ERROR
1668 %ERRNO
1669 %! Each element of "%!" has a true value only if $! is set to that
1670 value. For example, $!{ENOENT} is true if and only if the
1671 current value of $! is "ENOENT"; that is, if the most recent
1672 error was "No such file or directory" (or its moral equivalent:
1673 not all operating systems give that exact error, and certainly
1674 not all languages). The specific true value is not guaranteed,
1675 but in the past has generally been the numeric value of $!. To
1676 check if a particular key is meaningful on your system, use
1677 "exists $!{the_key}"; for a list of legal keys, use "keys %!".
1678 See Errno for more information, and also see "$!".
1679 This variable was added in Perl 5.005.
1681 $CHILD_ERROR
1683 $? The status returned by the last pipe close, backtick ("``")
1684 command, successful call to "wait()" or "waitpid()", or from
1685 the "system()" operator. This is just the 16-bit status word
1686 returned by the traditional Unix "wait()" system call (or else
1687 is made up to look like it). Thus, the exit value of the
1688 subprocess is really ("$? >> 8"), and "$? & 127" gives which
1689 signal, if any, the process died from, and "$? & 128" reports
1690 whether there was a core dump.
1691 Additionally, if the "h_errno" variable is supported in C, its
1693 value is returned via $? if any "gethost*()" function fails.
1694 If you have installed a signal handler for "SIGCHLD", the value
1696 of $? will usually be wrong outside that handler.
1697 Inside an "END" subroutine $? contains the value that is going
1699 to be given to "exit()". You can modify $? in an "END"
1700 subroutine to change the exit status of your program. For
1701 example:
1702 END {
1704 $? = 1 if $? == 255; # die would make it 255
1705 }
1706 Under VMS, the pragma "use vmsish 'status'" makes $? reflect
1708 the actual VMS exit status, instead of the default emulation of
1709 POSIX status; see "$?" in perlvms for details.
1710 Mnemonic: similar to sh and ksh.
1712 $EVAL_ERROR
1714 $@ The Perl error from the last "eval" operator, i.e. the last
1715 exception that was caught. For "eval BLOCK", this is either a
1716 runtime error message or the string or reference "die" was
1717 called with. The "eval STRING" form also catches syntax errors
1718 and other compile time exceptions.
1719 If no error occurs, "eval" sets $@ to the empty string.
1721 Warning messages are not collected in this variable. You can,
1723 however, set up a routine to process warnings by setting
1724 $SIG{__WARN__} as described in "%SIG".
1725 Mnemonic: Where was the error "at"?
1727 Variables related to the interpreter state
1729 These variables provide information about the current interpreter
1730 state.
1731 $COMPILING
1733 $^C The current value of the flag associated with the -c switch.
1734 Mainly of use with -MO=... to allow code to alter its behavior
1735 when being compiled, such as for example to "AUTOLOAD" at
1736 compile time rather than normal, deferred loading. Setting
1737 "$^C = 1" is similar to calling "B::minus_c".
1738 This variable was added in Perl v5.6.0.
1740 $DEBUGGING
1742 $^D The current value of the debugging flags. May be read or set.
1743 Like its command-line equivalent, you can use numeric or
1744 symbolic values, e.g. "$^D = 10" or "$^D = "st"". See
1745 "-Dnumber" in perlrun. The contents of this variable also
1746 affects the debugger operation. See "Debugger Internals" in
1747 perldebguts.
1748 Mnemonic: value of -D switch.
1750 ${^ENCODING}
1752 This variable is no longer supported.
1753 It used to hold the object reference to the "Encode" object
1755 that was used to convert the source code to Unicode.
1756 Its purpose was to allow your non-ASCII Perl scripts not to
1758 have to be written in UTF-8; this was useful before editors
1759 that worked on UTF-8 encoded text were common, but that was
1760 long ago. It caused problems, such as affecting the operation
1761 of other modules that weren't expecting it, causing general
1762 mayhem.
1763 If you need something like this functionality, it is
1765 recommended that use you a simple source filter, such as
1766 Filter::Encoding.
1767 If you are coming here because code of yours is being adversely
1769 affected by someone's use of this variable, you can usually
1770 work around it by doing this:
1771 local ${^ENCODING};
1773 near the beginning of the functions that are getting broken.
1775 This undefines the variable during the scope of execution of
1776 the including function.
1777 This variable was added in Perl 5.8.2 and removed in 5.26.0.
1779 Setting it to anything other than "undef" was made fatal in
1780 Perl 5.28.0.
1781 ${^GLOBAL_PHASE}
1783 The current phase of the perl interpreter.
1784 Possible values are:
1786 CONSTRUCT
1788 The "PerlInterpreter*" is being constructed via
1789 "perl_construct". This value is mostly there for
1790 completeness and for use via the underlying C variable
1791 "PL_phase". It's not really possible for Perl code to
1792 be executed unless construction of the interpreter is
1793 finished.
1794 START This is the global compile-time. That includes,
1796 basically, every "BEGIN" block executed directly or
1797 indirectly from during the compile-time of the top-
1798 level program.
1799 This phase is not called "BEGIN" to avoid confusion
1801 with "BEGIN"-blocks, as those are executed during
1802 compile-time of any compilation unit, not just the top-
1803 level program. A new, localised compile-time entered
1804 at run-time, for example by constructs as "eval "use
1805 SomeModule"" are not global interpreter phases, and
1806 therefore aren't reflected by "${^GLOBAL_PHASE}".
1807 CHECK Execution of any "CHECK" blocks.
1809 INIT Similar to "CHECK", but for "INIT"-blocks, not "CHECK"
1811 blocks.
1812 RUN The main run-time, i.e. the execution of
1814 "PL_main_root".
1815 END Execution of any "END" blocks.
1817 DESTRUCT
1819 Global destruction.
1820 Also note that there's no value for UNITCHECK-blocks. That's
1822 because those are run for each compilation unit individually,
1823 and therefore is not a global interpreter phase.
1824 Not every program has to go through each of the possible
1826 phases, but transition from one phase to another can only
1827 happen in the order described in the above list.
1828 An example of all of the phases Perl code can see:
1830 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1832 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1834 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1836 {
1838 package Print::Phase;
1839 sub new {
1841 my ($class, $time) = @_;
1842 return bless \$time, $class;
1843 }
1844 sub DESTROY {
1846 my $self = shift;
1847 print "$$self: ${^GLOBAL_PHASE}\n";
1848 }
1849 }
1850 print "run-time: ${^GLOBAL_PHASE}\n";
1852 my $runtime = Print::Phase->new(
1854 "lexical variables are garbage collected before END"
1855 );
1856 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1858 our $destruct = Print::Phase->new(
1860 "package variables are garbage collected after END"
1861 );
1862 This will print out
1864 compile-time: START
1866 check-time: CHECK
1867 init-time: INIT
1868 run-time: RUN
1869 lexical variables are garbage collected before END: RUN
1870 end-time: END
1871 package variables are garbage collected after END: DESTRUCT
1872 This variable was added in Perl 5.14.0.
1874 $^H WARNING: This variable is strictly for internal use only. Its
1876 availability, behavior, and contents are subject to change
1877 without notice.
1878 This variable contains compile-time hints for the Perl
1880 interpreter. At the end of compilation of a BLOCK the value of
1881 this variable is restored to the value when the interpreter
1882 started to compile the BLOCK.
1883 When perl begins to parse any block construct that provides a
1885 lexical scope (e.g., eval body, required file, subroutine body,
1886 loop body, or conditional block), the existing value of $^H is
1887 saved, but its value is left unchanged. When the compilation
1888 of the block is completed, it regains the saved value. Between
1889 the points where its value is saved and restored, code that
1890 executes within BEGIN blocks is free to change the value of
1891 $^H.
1892 This behavior provides the semantic of lexical scoping, and is
1894 used in, for instance, the "use strict" pragma.
1895 The contents should be an integer; different bits of it are
1897 used for different pragmatic flags. Here's an example:
1898 sub add_100 { $^H |= 0x100 }
1900 sub foo {
1902 BEGIN { add_100() }
1903 bar->baz($boon);
1904 }
1905 Consider what happens during execution of the BEGIN block. At
1907 this point the BEGIN block has already been compiled, but the
1908 body of "foo()" is still being compiled. The new value of $^H
1909 will therefore be visible only while the body of "foo()" is
1910 being compiled.
1911 Substitution of "BEGIN { add_100() }" block with:
1913 BEGIN { require strict; strict->import('vars') }
1915 demonstrates how "use strict 'vars'" is implemented. Here's a
1917 conditional version of the same lexical pragma:
1918 BEGIN {
1920 require strict; strict->import('vars') if $condition
1921 }
1922 This variable was added in Perl 5.003.
1924 %^H The "%^H" hash provides the same scoping semantic as $^H. This
1926 makes it useful for implementation of lexically scoped pragmas.
1927 See perlpragma. All the entries are stringified when accessed
1928 at runtime, so only simple values can be accommodated. This
1929 means no pointers to objects, for example.
1930 When putting items into "%^H", in order to avoid conflicting
1932 with other users of the hash there is a convention regarding
1933 which keys to use. A module should use only keys that begin
1934 with the module's name (the name of its main package) and a "/"
1935 character. For example, a module "Foo::Bar" should use keys
1936 such as "Foo::Bar/baz".
1937 This variable was added in Perl v5.6.0.
1939 ${^OPEN}
1941 An internal variable used by PerlIO. A string in two parts,
1942 separated by a "\0" byte, the first part describes the input
1943 layers, the second part describes the output layers.
1944 This variable was added in Perl v5.8.0.
1946 $PERLDB
1948 $^P The internal variable for debugging support. The meanings of
1949 the various bits are subject to change, but currently indicate:
1950 0x01 Debug subroutine enter/exit.
1952 0x02 Line-by-line debugging. Causes "DB::DB()" subroutine to
1954 be called for each statement executed. Also causes
1955 saving source code lines (like 0x400).
1956 0x04 Switch off optimizations.
1958 0x08 Preserve more data for future interactive inspections.
1960 0x10 Keep info about source lines on which a subroutine is
1962 defined.
1963 0x20 Start with single-step on.
1965 0x40 Use subroutine address instead of name when reporting.
1967 0x80 Report "goto &subroutine" as well.
1969 0x100 Provide informative "file" names for evals based on the
1971 place they were compiled.
1972 0x200 Provide informative names to anonymous subroutines based
1974 on the place they were compiled.
1975 0x400 Save source code lines into "@{"_<$filename"}".
1977 0x800 When saving source, include evals that generate no
1979 subroutines.
1980 0x1000
1982 When saving source, include source that did not compile.
1983 Some bits may be relevant at compile-time only, some at run-
1985 time only. This is a new mechanism and the details may change.
1986 See also perldebguts.
1987 ${^TAINT}
1989 Reflects if taint mode is on or off. 1 for on (the program was
1990 run with -T), 0 for off, -1 when only taint warnings are
1991 enabled (i.e. with -t or -TU).
1992 This variable is read-only.
1994 This variable was added in Perl v5.8.0.
1996 ${^SAFE_LOCALES}
1998 Reflects if safe locale operations are available to this perl
1999 (when the value is 1) or not (the value is 0). This variable
2000 is always 1 if the perl has been compiled without threads. It
2001 is also 1 if this perl is using thread-safe locale operations.
2002 Note that an individual thread may choose to use the global
2003 locale (generally unsafe) by calling "switch_to_global_locale"
2004 in perlapi. This variable currently is still set to 1 in such
2005 threads.
2006 This variable is read-only.
2008 This variable was added in Perl v5.28.0.
2010 ${^UNICODE}
2012 Reflects certain Unicode settings of Perl. See perlrun
2013 documentation for the "-C" switch for more information about
2014 the possible values.
2015 This variable is set during Perl startup and is thereafter
2017 read-only.
2018 This variable was added in Perl v5.8.2.
2020 ${^UTF8CACHE}
2022 This variable controls the state of the internal UTF-8 offset
2023 caching code. 1 for on (the default), 0 for off, -1 to debug
2024 the caching code by checking all its results against linear
2025 scans, and panicking on any discrepancy.
2026 This variable was added in Perl v5.8.9. It is subject to
2028 change or removal without notice, but is currently used to
2029 avoid recalculating the boundaries of multi-byte UTF-8-encoded
2030 characters.
2031 ${^UTF8LOCALE}
2033 This variable indicates whether a UTF-8 locale was detected by
2034 perl at startup. This information is used by perl when it's in
2035 adjust-utf8ness-to-locale mode (as when run with the "-CL"
2036 command-line switch); see perlrun for more info on this.
2037 This variable was added in Perl v5.8.8.
2039 Deprecated and removed variables
2041 Deprecating a variable announces the intent of the perl maintainers to
2042 eventually remove the variable from the language. It may still be
2043 available despite its status. Using a deprecated variable triggers a
2044 warning.
2045 Once a variable is removed, its use triggers an error telling you the
2047 variable is unsupported.
2048 See perldiag for details about error messages.
2050 $# $# was a variable that could be used to format printed numbers.
2052 After a deprecation cycle, its magic was removed in Perl
2053 v5.10.0 and using it now triggers a warning: "$# is no longer
2054 supported".
2055 This is not the sigil you use in front of an array name to get
2057 the last index, like $#array. That's still how you get the
2058 last index of an array in Perl. The two have nothing to do
2059 with each other.
2060 Deprecated in Perl 5.
2062 Removed in Perl v5.10.0.
2064 $* $* was a variable that you could use to enable multiline
2066 matching. After a deprecation cycle, its magic was removed in
2067 Perl v5.10.0. Using it now triggers a warning: "$* is no
2068 longer supported". You should use the "/s" and "/m" regexp
2069 modifiers instead.
2070 Deprecated in Perl 5.
2072 Removed in Perl v5.10.0.
2074 $[ This variable stores the index of the first element in an
2076 array, and of the first character in a substring. The default
2077 is 0, but you could theoretically set it to 1 to make Perl
2078 behave more like awk (or Fortran) when subscripting and when
2079 evaluating the index() and substr() functions.
2080 As of release 5 of Perl, assignment to $[ is treated as a
2082 compiler directive, and cannot influence the behavior of any
2083 other file. (That's why you can only assign compile-time
2084 constants to it.) Its use is highly discouraged.
2085 Prior to Perl v5.10.0, assignment to $[ could be seen from
2087 outer lexical scopes in the same file, unlike other compile-
2088 time directives (such as strict). Using local() on it would
2089 bind its value strictly to a lexical block. Now it is always
2090 lexically scoped.
2091 As of Perl v5.16.0, it is implemented by the arybase module.
2093 As of Perl v5.30.0, or under "use v5.16", or "no feature
2095 "array_base"", $[ no longer has any effect, and always contains
2096 0. Assigning 0 to it is permitted, but any other value will
2097 produce an error.
2098 Mnemonic: [ begins subscripts.
2100 Deprecated in Perl v5.12.0.
2102perl v5.30.2 2020-03-27 PERLVAR(1)