3 perlvar - Perl predefined variables
7 =head2 The Syntax of Variable Names
9 Variable names in Perl can have several formats. Usually, they
10 must begin with a letter or underscore, in which case they can be
11 arbitrarily long (up to an internal limit of 251 characters) and
12 may contain letters, digits, underscores, or the special sequence
13 C<::> or C<'>. In this case, the part before the last C<::> or
14 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
15 A Unicode letter that is not ASCII is not considered to be a letter
16 unless S<C<"use utf8">> is in effect, and somewhat more complicated
17 rules apply; see L<perldata/Identifier parsing> for details.
19 Perl variable names may also be a sequence of digits, a single
20 punctuation character, or the two-character sequence: C<^> (caret or
21 CIRCUMFLEX ACCENT) followed by any one of the characters C<[][A-Z^_?\]>.
22 These names are all reserved for
23 special uses by Perl; for example, the all-digits names are used
24 to hold data captured by backreferences after a regular expression
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 C<${^Foo}>;
29 the braces are not optional. C<${^Foo}> denotes the scalar variable
30 whose name is considered to be a control-C<F> followed by two C<o>'s.
32 reserved for future special uses by Perl, except for the ones that
33 begin with C<^_> (caret-underscore). No
34 name that begins with C<^_> will acquire a special
35 meaning in any future version of Perl; such names may therefore be
36 used safely in programs. C<$^_> itself, however, I<is> reserved.
38 Perl identifiers that begin with digits or
39 punctuation characters are exempt from the effects of the C<package>
40 declaration and are always forced to be in package C<main>; they are
41 also exempt from C<strict 'vars'> errors. A few other names are also
50 In particular, the special C<${^_XYZ}> variables are always taken
51 to be in package C<main>, regardless of any C<package> declarations
54 =head1 SPECIAL VARIABLES
56 The following names have special meaning to Perl. Most punctuation
57 names have reasonable mnemonics, or analogs in the shells.
58 Nevertheless, if you wish to use long variable names, you need only say:
62 at the top of your program. This aliases all the short names to the long
63 names in the current package. Some even have medium names, generally
64 borrowed from B<awk>. For more info, please see L<English>.
66 Before you continue, note the sort order for variables. In general, we
67 first list the variables in case-insensitive, almost-lexigraphical
68 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
69 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
70 For variables with the same identifier, we list it in order of scalar,
71 array, hash, and bareword.
73 =head2 General Variables
82 The default input and pattern-searching space. The following pairs are
85 while (<>) {...} # equivalent only in while!
86 while (defined($_ = <>)) {...}
97 Here are the places where Perl will assume C<$_> even if you don't use it:
103 The following functions use C<$_> as a default argument:
105 abs, alarm, chomp, chop, chr, chroot,
106 cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
107 lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
108 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
109 rmdir, say, sin, split (for its second
110 argument), sqrt, stat, study, uc, ucfirst,
115 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
120 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
121 when used without an C<=~> operator.
125 The default iterator variable in a C<foreach> loop if no other
126 variable is supplied.
130 The implicit iterator variable in the C<grep()> and C<map()> functions.
134 The implicit variable of C<given()>.
138 The default place to put the next value or input record
139 when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
140 operation's result is tested by itself as the sole criterion of a C<while>
141 test. Outside a C<while> test, this will not happen.
145 C<$_> is a global variable.
147 However, between perl v5.10.0 and v5.24.0, it could be used lexically by
148 writing C<my $_>. Making C<$_> refer to the global C<$_> in the same scope
149 was then possible with C<our $_>. This experimental feature was removed and is
150 now a fatal error, but you may encounter it in older code.
152 Mnemonic: underline is understood in certain operations.
159 Within a subroutine the array C<@_> contains the parameters passed to
160 that subroutine. Inside a subroutine, C<@_> is the default array for
161 the array operators C<pop> and C<shift>.
165 =item $LIST_SEPARATOR
168 X<$"> X<$LIST_SEPARATOR>
170 When an array or an array slice is interpolated into a double-quoted
171 string or a similar context such as C</.../>, its elements are
172 separated by this value. Default is a space. For example, this:
174 print "The array is: @array\n";
176 is equivalent to this:
178 print "The array is: " . join($", @array) . "\n";
180 Mnemonic: works in double-quoted context.
187 X<$$> X<$PID> X<$PROCESS_ID>
189 The process number of the Perl running this script. Though you I<can> set
190 this variable, doing so is generally discouraged, although it can be
191 invaluable for some testing purposes. It will be reset automatically
192 across C<fork()> calls.
194 Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
195 would emulate POSIX semantics on Linux systems using LinuxThreads, a
196 partial implementation of POSIX Threads that has since been superseded
197 by the Native POSIX Thread Library (NPTL).
199 LinuxThreads is now obsolete on Linux, and caching C<getpid()>
200 like this made embedding perl unnecessarily complex (since you'd have
201 to manually update the value of $$), so now C<$$> and C<getppid()>
202 will always return the same values as the underlying C library.
204 Debian GNU/kFreeBSD systems also used LinuxThreads up until and
205 including the 6.0 release, but after that moved to FreeBSD thread
206 semantics, which are POSIX-like.
208 To see if your system is affected by this discrepancy check if
209 C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
210 value. NTPL threads preserve the POSIX semantics.
212 Mnemonic: same as shells.
217 X<$0> X<$PROGRAM_NAME>
219 Contains the name of the program being executed.
221 On some (but not all) operating systems assigning to C<$0> modifies
222 the argument area that the C<ps> program sees. On some platforms you
223 may have to use special C<ps> options or a different C<ps> to see the
224 changes. Modifying the C<$0> is more useful as a way of indicating the
225 current program state than it is for hiding the program you're
228 Note that there are platform-specific limitations on the maximum
229 length of C<$0>. In the most extreme case it may be limited to the
230 space occupied by the original C<$0>.
232 In some platforms there may be arbitrary amount of padding, for
233 example space characters, after the modified name as shown by C<ps>.
234 In some platforms this padding may extend all the way to the original
235 length of the argument area, no matter what you do (this is the case
236 for example with Linux 2.2).
238 Note for BSD users: setting C<$0> does not completely remove "perl"
239 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
240 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
241 and the " (perl)" suffix are shown depends on your exact BSD variant
242 and version). This is an operating system feature, Perl cannot help it.
244 In multithreaded scripts Perl coordinates the threads so that any
245 thread may modify its copy of the C<$0> and the change becomes visible
246 to ps(1) (assuming the operating system plays along). Note that
247 the view of C<$0> the other threads have will not change since they
248 have their own copies of it.
250 If the program has been given to perl via the switches C<-e> or C<-E>,
251 C<$0> will contain the string C<"-e">.
253 On Linux as of perl v5.14.0 the legacy process name will be set with
254 C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
255 perl has done since version 4.000. Now system utilities that read the
256 legacy process name such as ps, top and killall will recognize the
257 name you set when assigning to C<$0>. The string you supply will be
258 cut off at 16 bytes, this is a limitation imposed by Linux.
260 Mnemonic: same as B<sh> and B<ksh>.
267 X<$(> X<$GID> X<$REAL_GROUP_ID>
269 The real gid of this process. If you are on a machine that supports
270 membership in multiple groups simultaneously, gives a space separated
271 list of groups you are in. The first number is the one returned by
272 C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
273 the same as the first number.
275 However, a value assigned to C<$(> must be a single number used to
276 set the real gid. So the value given by C<$(> should I<not> be assigned
277 back to C<$(> without being forced numeric, such as by adding zero. Note
278 that this is different to the effective gid (C<$)>) which does take a
281 You can change both the real gid and the effective gid at the same
282 time by using C<POSIX::setgid()>. Changes
283 to C<$(> require a check to C<$!>
284 to detect any possible errors after an attempted change.
286 Mnemonic: parentheses are used to I<group> things. The real gid is the
287 group you I<left>, if you're running setgid.
289 =item $EFFECTIVE_GROUP_ID
294 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
296 The effective gid of this process. If you are on a machine that
297 supports membership in multiple groups simultaneously, gives a space
298 separated list of groups you are in. The first number is the one
299 returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
300 one of which may be the same as the first number.
302 Similarly, a value assigned to C<$)> must also be a space-separated
303 list of numbers. The first number sets the effective gid, and
304 the rest (if any) are passed to C<setgroups()>. To get the effect of an
305 empty list for C<setgroups()>, just repeat the new effective gid; that is,
306 to force an effective gid of 5 and an effectively empty C<setgroups()>
307 list, say C< $) = "5 5" >.
309 You can change both the effective gid and the real gid at the same
310 time by using C<POSIX::setgid()> (use only a single numeric argument).
311 Changes to C<$)> require a check to C<$!> to detect any possible errors
312 after an attempted change.
314 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
315 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
316 and C<$)> can be swapped only on machines supporting C<setregid()>.
318 Mnemonic: parentheses are used to I<group> things. The effective gid
319 is the group that's I<right> for you, if you're running setgid.
326 X<< $< >> X<$UID> X<$REAL_USER_ID>
328 The real uid of this process. You can change both the real uid and the
329 effective uid at the same time by using C<POSIX::setuid()>. Since
330 changes to C<< $< >> require a system call, check C<$!> after a change
331 attempt to detect any possible errors.
333 Mnemonic: it's the uid you came I<from>, if you're running setuid.
335 =item $EFFECTIVE_USER_ID
340 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
342 The effective uid of this process. For example:
344 $< = $>; # set real to effective uid
345 ($<,$>) = ($>,$<); # swap real and effective uids
347 You can change both the effective uid and the real uid at the same
348 time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
349 to C<$!> to detect any possible errors after an attempted change.
351 C<< $< >> and C<< $> >> can be swapped only on machines
352 supporting C<setreuid()>.
354 Mnemonic: it's the uid you went I<to>, if you're running setuid.
356 =item $SUBSCRIPT_SEPARATOR
361 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
363 The subscript separator for multidimensional array emulation. If you
364 refer to a hash element as
370 $foo{join($;, $x, $y, $z)}
374 @foo{$x,$y,$z} # a slice--note the @
378 ($foo{$x},$foo{$y},$foo{$z})
380 Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
381 binary data there might not be any safe value for C<$;>.
383 Consider using "real" multidimensional arrays as described
386 Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
393 Special package variables when using C<sort()>, see L<perlfunc/sort>.
394 Because of this specialness C<$a> and C<$b> don't need to be declared
395 (using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
396 pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
397 be able to use them in the C<sort()> comparison block or function.
402 The hash C<%ENV> contains your current environment. Setting a
403 value in C<ENV> changes the environment for any child processes
404 you subsequently C<fork()> off.
406 As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
410 if( ref $ENV{'bar'} ) {
411 say "Pre 5.18.0 Behaviour";
413 say "Post 5.18.0 Behaviour";
416 Previously, only child processes received stringified values:
421 # Always printed 'non ref'
423 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
425 This happens because you can't really share arbitrary data structures with
428 =item $OLD_PERL_VERSION
431 X<$]> X<$OLD_PERL_VERSION>
433 The revision, version, and subversion of the Perl interpreter, represented
434 as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
435 is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
437 This variable can be used to determine whether the Perl interpreter
438 executing a script is in the right range of versions:
440 warn "No PerlIO!\n" if $] lt '5.008';
442 When comparing C<$]>, string comparison operators are B<highly
443 recommended>. The inherent limitations of binary floating point
444 representation can sometimes lead to incorrect comparisons for some
445 numbers on some architectures.
447 See also the documentation of C<use VERSION> and C<require VERSION>
448 for a convenient way to fail if the running Perl interpreter is too old.
450 See L</$^V> for a representation of the Perl version as a L<version>
451 object, which allows more flexible string comparisons.
453 The main advantage of C<$]> over C<$^V> is that it works the same on any
454 version of Perl. The disadvantages are that it can't easily be compared
455 to versions in other formats (e.g. literal v-strings, "v1.2.3" or
456 version objects) and numeric comparisons can occasionally fail; it's good
457 for string literal version checks and bad for comparing to a variable
458 that hasn't been sanity-checked.
460 The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
461 reasons but its use is discouraged. (If your reason to use C<$]> is to
462 run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
465 Mnemonic: Is this version of perl in the right bracket?
470 X<$^F> X<$SYSTEM_FD_MAX>
472 The maximum system file descriptor, ordinarily 2. System file
473 descriptors are passed to C<exec()>ed processes, while higher file
474 descriptors are not. Also, during an
475 C<open()>, system file descriptors are
476 preserved even if the C<open()> fails (ordinary file descriptors are
477 closed before the C<open()> is attempted). The close-on-exec
478 status of a file descriptor will be decided according to the value of
479 C<$^F> when the corresponding file, pipe, or socket was opened, not the
480 time of the C<exec()>.
485 The array C<@F> contains the fields of each line read in when autosplit
486 mode is turned on. See L<perlrun> for the B<-a> switch. This array
487 is package-specific, and must be declared or given a full package name
488 if not in package main when running under C<strict 'vars'>.
493 The array C<@INC> contains the list of places that the C<do EXPR>,
494 C<require>, or C<use> constructs look for their library files. It
495 initially consists of the arguments to any B<-I> command-line
496 switches, followed by the default Perl library, probably
497 F</usr/local/lib/perl>, followed by ".", to represent the current
498 directory. ("." will not be appended if taint checks are enabled,
499 either by C<-T> or by C<-t>, or if configured not to do so by the
500 C<-Ddefault_inc_excludes_dot> compile time option.) If you need to
501 modify this at runtime, you should use the C<use lib> pragma to get
502 the machine-dependent library properly loaded also:
504 use lib '/mypath/libdir/';
507 You can also insert hooks into the file inclusion system by putting Perl
508 code directly into C<@INC>. Those hooks may be subroutine references,
509 array references or blessed objects. See L<perlfunc/require> for details.
514 The hash C<%INC> contains entries for each filename included via the
515 C<do>, C<require>, or C<use> operators. The key is the filename
516 you specified (with module names converted to pathnames), and the
517 value is the location of the file found. The C<require>
518 operator uses this hash to determine whether a particular file has
519 already been included.
521 If the file was loaded via a hook (e.g. a subroutine reference, see
522 L<perlfunc/require> for a description of these hooks), this hook is
523 by default inserted into C<%INC> in place of a filename. Note, however,
524 that the hook may have set the C<%INC> entry by itself to provide some more
530 X<$^I> X<$INPLACE_EDIT>
532 The current value of the inplace-edit extension. Use C<undef> to disable
535 Mnemonic: value of B<-i> switch.
540 Each package contains a special array called C<@ISA> which contains a list
541 of that class's parent classes, if any. This array is simply a list of
542 scalars, each of which is a string that corresponds to a package name. The
543 array is examined when Perl does method resolution, which is covered in
546 To load packages while adding them to C<@ISA>, see the L<parent> pragma. The
547 discouraged L<base> pragma does this as well, but should not be used except
548 when compatibility with the discouraged L<fields> pragma is required.
553 By default, running out of memory is an untrappable, fatal error.
554 However, if suitably built, Perl can use the contents of C<$^M>
555 as an emergency memory pool after C<die()>ing. Suppose that your Perl
556 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
559 $^M = 'a' x (1 << 16);
561 would allocate a 64K buffer for use in an emergency. See the
562 F<INSTALL> file in the Perl distribution for information on how to
563 add custom C compilation flags when compiling perl. To discourage casual
564 use of this advanced feature, there is no L<English|English> long name for
567 This variable was added in Perl 5.004.
574 The name of the operating system under which this copy of Perl was
575 built, as determined during the configuration process. For examples
576 see L<perlport/PLATFORMS>.
578 The value is identical to C<$Config{'osname'}>. See also L<Config>
579 and the B<-V> command-line switch documented in L<perlrun>.
581 In Windows platforms, C<$^O> is not very helpful: since it is always
582 C<MSWin32>, it doesn't tell the difference between
583 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
584 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
585 between the variants.
587 This variable was added in Perl 5.003.
592 The hash C<%SIG> contains signal handlers for signals. For example:
594 sub handler { # 1st argument is signal name
596 print "Caught a SIG$sig--shutting down\n";
601 $SIG{'INT'} = \&handler;
602 $SIG{'QUIT'} = \&handler;
604 $SIG{'INT'} = 'DEFAULT'; # restore default action
605 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
607 Using a value of C<'IGNORE'> usually has the effect of ignoring the
608 signal, except for the C<CHLD> signal. See L<perlipc> for more about
611 Here are some other examples:
613 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
615 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
617 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
618 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
621 Be sure not to use a bareword as the name of a signal handler,
622 lest you inadvertently call it.
624 If your system has the C<sigaction()> function then signal handlers
625 are installed using it. This means you get reliable signal handling.
627 The default delivery policy of signals changed in Perl v5.8.0 from
628 immediate (also known as "unsafe") to deferred, also known as "safe
629 signals". See L<perlipc> for more information.
631 Certain internal hooks can be also set using the C<%SIG> hash. The
632 routine indicated by C<$SIG{__WARN__}> is called when a warning
633 message is about to be printed. The warning message is passed as the
634 first argument. The presence of a C<__WARN__> hook causes the
635 ordinary printing of warnings to C<STDERR> to be suppressed. You can
636 use this to save warnings in a variable, or turn warnings into fatal
639 local $SIG{__WARN__} = sub { die $_[0] };
642 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
643 disable warnings using the empty subroutine:
645 local $SIG{__WARN__} = sub {};
647 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
648 exception is about to be thrown. The error message is passed as the
649 first argument. When a C<__DIE__> hook routine returns, the exception
650 processing continues as it would have in the absence of the hook,
651 unless the hook routine itself exits via a C<goto &sub>, a loop exit,
652 or a C<die()>. The C<__DIE__> handler is explicitly disabled during
653 the call, so that you can die from a C<__DIE__> handler. Similarly
656 The C<$SIG{__DIE__}> hook is called even inside an C<eval()>. It was
657 never intended to happen this way, but an implementation glitch made
658 this possible. This used to be deprecated, as it allowed strange action
659 at a distance like rewriting a pending exception in C<$@>. Plans to
660 rectify this have been scrapped, as users found that rewriting a
661 pending exception is actually a useful feature, and not a bug.
663 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
664 may be called to report (probable) errors found by the parser. In such
665 a case the parser may be in inconsistent state, so any attempt to
666 evaluate Perl code from such a handler will probably result in a
667 segfault. This means that warnings or errors that result from parsing
668 Perl should be used with extreme caution, like this:
670 require Carp if defined $^S;
671 Carp::confess("Something wrong") if defined &Carp::confess;
672 die "Something wrong, but could not load Carp to give "
674 . "To see backtrace try starting Perl with -MCarp switch";
676 Here the first line will load C<Carp> I<unless> it is the parser who
677 called the handler. The second line will print backtrace and die if
678 C<Carp> was available. The third line will be executed only if C<Carp> was
681 Having to even think about the C<$^S> variable in your exception
682 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
683 invites grievous and difficult to track down errors. Avoid it
684 and use an C<END{}> or CORE::GLOBAL::die override instead.
686 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
687 L<warnings> for additional information.
694 The time at which the program began running, in seconds since the
695 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
696 and B<-C> filetests are based on this value.
701 X<$^V> X<$PERL_VERSION>
703 The revision, version, and subversion of the Perl interpreter,
704 represented as a L<version> object.
706 This variable first appeared in perl v5.6.0; earlier versions of perl
707 will see an undefined value. Before perl v5.10.0 C<$^V> was represented
708 as a v-string rather than a L<version> object.
710 C<$^V> can be used to determine whether the Perl interpreter executing
711 a script is in the right range of versions. For example:
713 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
715 While version objects overload stringification, to portably convert
716 C<$^V> into its string representation, use C<sprintf()>'s C<"%vd">
717 conversion, which works for both v-strings or version objects:
719 printf "version is v%vd\n", $^V; # Perl's version
721 See the documentation of C<use VERSION> and C<require VERSION>
722 for a convenient way to fail if the running Perl interpreter is too old.
724 See also C<L</$]>> for a decimal representation of the Perl version.
726 The main advantage of C<$^V> over C<$]> is that, for Perl v5.10.0 or
727 later, it overloads operators, allowing easy comparison against other
728 version representations (e.g. decimal, literal v-string, "v1.2.3", or
729 objects). The disadvantage is that prior to v5.10.0, it was only a
730 literal v-string, which can't be easily printed or compared, whereas
731 the behavior of C<$]> is unchanged on all versions of Perl.
733 Mnemonic: use ^V for a version object.
735 =item ${^WIN32_SLOPPY_STAT}
736 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
738 If this variable is set to a true value, then C<stat()> on Windows will
739 not try to open the file. This means that the link count cannot be
740 determined and file attributes may be out of date if additional
741 hardlinks to the file exist. On the other hand, not opening the file
742 is considerably faster, especially for files on network drives.
744 This variable could be set in the F<sitecustomize.pl> file to
745 configure the local Perl installation to use "sloppy" C<stat()> by
746 default. See the documentation for B<-f> in
747 L<perlrun|perlrun/"Command Switches"> for more information about site
750 This variable was added in Perl v5.10.0.
752 =item $EXECUTABLE_NAME
755 X<$^X> X<$EXECUTABLE_NAME>
757 The name used to execute the current copy of Perl, from C's
758 C<argv[0]> or (where supported) F</proc/self/exe>.
760 Depending on the host operating system, the value of C<$^X> may be
761 a relative or absolute pathname of the perl program file, or may
762 be the string used to invoke perl but not the pathname of the
763 perl program file. Also, most operating systems permit invoking
764 programs that are not in the PATH environment variable, so there
765 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
766 value may or may not include a version number.
768 You usually can use the value of C<$^X> to re-invoke an independent
769 copy of the same perl that is currently running, e.g.,
771 @first_run = `$^X -le "print int rand 100 for 1..100"`;
773 But recall that not all operating systems support forking or
774 capturing of the output of commands, so this complex statement
777 It is not safe to use the value of C<$^X> as a path name of a file,
778 as some operating systems that have a mandatory suffix on
779 executable files do not require use of the suffix when invoking
780 a command. To convert the value of C<$^X> to a path name, use the
781 following statements:
783 # Build up a set of file names (not command names).
787 $this_perl .= $Config{_exe}
788 unless $this_perl =~ m/$Config{_exe}$/i;
791 Because many operating systems permit anyone with read access to
792 the Perl program file to make a copy of it, patch the copy, and
793 then execute the copy, the security-conscious Perl programmer
794 should take care to invoke the installed copy of perl, not the
795 copy referenced by C<$^X>. The following statements accomplish
796 this goal, and produce a pathname that can be invoked as a
797 command or referenced as a file.
800 my $secure_perl_path = $Config{perlpath};
802 $secure_perl_path .= $Config{_exe}
803 unless $secure_perl_path =~ m/$Config{_exe}$/i;
808 =head2 Variables related to regular expressions
810 Most of the special variables related to regular expressions are side
811 effects. Perl sets these variables when it has a successful match, so
812 you should check the match result before using them. For instance:
814 if( /P(A)TT(ER)N/ ) {
815 print "I found $1 and $2\n";
818 These variables are read-only and dynamically-scoped, unless we note
821 The dynamic nature of the regular expression variables means that
822 their value is limited to the block that they are in, as demonstrated
825 my $outer = 'Wallace and Grommit';
826 my $inner = 'Mutt and Jeff';
828 my $pattern = qr/(\S+) and (\S+)/;
830 sub show_n { print "\$1 is $1; \$2 is $2\n" }
834 show_n() if $outer =~ m/$pattern/;
837 show_n() if $inner =~ m/$pattern/;
843 The output shows that while in the C<OUTER> block, the values of C<$1>
844 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
845 block, the values of C<$1> and C<$2> are from the match against
846 C<$inner>, but only until the end of the block (i.e. the dynamic
847 scope). After the C<INNER> block completes, the values of C<$1> and
848 C<$2> return to the values for the match against C<$outer> even though
849 we have not made another match:
851 $1 is Wallace; $2 is Grommit
852 $1 is Mutt; $2 is Jeff
853 $1 is Wallace; $2 is Grommit
855 =head3 Performance issues
857 Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
858 or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
859 all subsequent successful pattern matches to make a copy of the matched
860 string, in case the code might subsequently access one of those variables.
861 This imposed a considerable performance penalty across the whole program,
862 so generally the use of these variables has been discouraged.
864 In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
865 supply the indices of successful matches. So you could for example do
870 print $`, $&, $'; # bad: performance hit
872 print # good: no performance hit
873 substr($str, 0, $-[0]),
874 substr($str, $-[0], $+[0]-$-[0]),
877 In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
878 C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
879 you to suffer the penalties only on patterns marked with C</p>.
881 In Perl 5.18.0 onwards, perl started noting the presence of each of the
882 three variables separately, and only copied that part of the string
885 $`; $&; "abcdefgh" =~ /d/
887 perl would only copy the "abcd" part of the string. That could make a big
888 difference in something like
890 $str = 'x' x 1_000_000;
892 $str =~ /x/g # one char copied a million times, not a million chars
894 In Perl 5.20.0 a new copy-on-write system was enabled by default, which
895 finally fixes all performance issues with these three variables, and makes
896 them safe to use anywhere.
898 The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
899 find uses of these problematic match variables in your code.
903 =item $<I<digits>> ($1, $2, ...)
904 X<$1> X<$2> X<$3> X<$I<digits>>
906 Contains the subpattern from the corresponding set of capturing
907 parentheses from the last successful pattern match, not counting patterns
908 matched in nested blocks that have been exited already.
910 Note there is a distinction between a capture buffer which matches
911 the empty string a capture buffer which is optional. Eg, C<(x?)> and
912 C<(x)?> The latter may be undef, the former not.
914 These variables are read-only and dynamically-scoped.
916 Mnemonic: like \digits.
919 X<@{^CAPTURE}> X<@^CAPTURE>
921 An array which exposes the contents of the capture buffers, if any, of
922 the last successful pattern match, not counting patterns matched
923 in nested blocks that have been exited already.
925 Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1 index
926 is equivalent to $2, etc.
928 if ("foal"=~/(.)(.)(.)(.)/) {
929 print join "-", @{^CAPTURE};
932 should output "f-o-a-l".
934 See also L</$I<digits>>, L</%{^CAPTURE}> and L</%{^CAPTURE_ALL}>.
936 Note that unlike most other regex magic variables there is no single
937 letter equivalent to C<@{^CAPTURE}>.
939 This variable was added in 5.25.7
946 The string matched by the last successful pattern match (not counting
947 any matches hidden within a BLOCK or C<eval()> enclosed by the current
950 See L</Performance issues> above for the serious performance implications
951 of using this variable (even once) in your code.
953 This variable is read-only and dynamically-scoped.
955 Mnemonic: like C<&> in some editors.
960 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
961 performance penalty associated with that variable.
963 See L</Performance issues> above.
965 In Perl v5.18 and earlier, it is only guaranteed
966 to return a defined value when the pattern was compiled or executed with
967 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
968 C<${^MATCH}> does the same thing as C<$MATCH>.
970 This variable was added in Perl v5.10.0.
972 This variable is read-only and dynamically-scoped.
977 X<$`> X<$PREMATCH> X<${^PREMATCH}>
979 The string preceding whatever was matched by the last successful
980 pattern match, not counting any matches hidden within a BLOCK or C<eval>
981 enclosed by the current BLOCK.
983 See L</Performance issues> above for the serious performance implications
984 of using this variable (even once) in your code.
986 This variable is read-only and dynamically-scoped.
988 Mnemonic: C<`> often precedes a quoted string.
991 X<$`> X<${^PREMATCH}>
993 This is similar to C<$`> ($PREMATCH) except that it does not incur the
994 performance penalty associated with that variable.
996 See L</Performance issues> above.
998 In Perl v5.18 and earlier, it is only guaranteed
999 to return a defined value when the pattern was compiled or executed with
1000 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1001 C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
1003 This variable was added in Perl v5.10.0.
1005 This variable is read-only and dynamically-scoped.
1010 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
1012 The string following whatever was matched by the last successful
1013 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1014 enclosed by the current BLOCK). Example:
1016 local $_ = 'abcdefghi';
1018 print "$`:$&:$'\n"; # prints abc:def:ghi
1020 See L</Performance issues> above for the serious performance implications
1021 of using this variable (even once) in your code.
1023 This variable is read-only and dynamically-scoped.
1025 Mnemonic: C<'> often follows a quoted string.
1028 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
1030 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1031 performance penalty associated with that variable.
1033 See L</Performance issues> above.
1035 In Perl v5.18 and earlier, it is only guaranteed
1036 to return a defined value when the pattern was compiled or executed with
1037 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1038 C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
1040 This variable was added in Perl v5.10.0.
1042 This variable is read-only and dynamically-scoped.
1044 =item $LAST_PAREN_MATCH
1047 X<$+> X<$LAST_PAREN_MATCH>
1049 The text matched by the last bracket of the last successful search pattern.
1050 This is useful if you don't know which one of a set of alternative patterns
1051 matched. For example:
1053 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
1055 This variable is read-only and dynamically-scoped.
1057 Mnemonic: be positive and forward looking.
1059 =item $LAST_SUBMATCH_RESULT
1062 X<$^N> X<$LAST_SUBMATCH_RESULT>
1064 The text matched by the used group most-recently closed (i.e. the group
1065 with the rightmost closing parenthesis) of the last successful search
1068 This is primarily used inside C<(?{...})> blocks for examining text
1069 recently matched. For example, to effectively capture text to a variable
1070 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1072 (?:(...)(?{ $var = $^N }))
1074 By setting and then using C<$var> in this way relieves you from having to
1075 worry about exactly which numbered set of parentheses they are.
1077 This variable was added in Perl v5.8.0.
1079 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1081 =item @LAST_MATCH_END
1084 X<@+> X<@LAST_MATCH_END>
1086 This array holds the offsets of the ends of the last successful
1087 submatches in the currently active dynamic scope. C<$+[0]> is
1088 the offset into the string of the end of the entire match. This
1089 is the same value as what the C<pos> function returns when called
1090 on the variable that was matched against. The I<n>th element
1091 of this array holds the offset of the I<n>th submatch, so
1092 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
1093 past where C<$2> ends, and so on. You can use C<$#+> to determine
1094 how many subgroups were in the last successful match. See the
1095 examples given for the C<@-> variable.
1097 This variable was added in Perl v5.6.0.
1101 =item %LAST_PAREN_MATCH
1104 X<%+> X<%LAST_PAREN_MATCH> X<%{^CAPTURE}>
1106 Similar to C<@+>, the C<%+> hash allows access to the named capture
1107 buffers, should they exist, in the last successful match in the
1108 currently active dynamic scope.
1110 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1112 'foo' =~ /(?<foo>foo)/;
1114 The keys of the C<%+> hash list only the names of buffers that have
1115 captured (and that are thus associated to defined values).
1117 If multiple distinct capture groups have the same name, then
1118 C<$+{NAME}> will refer to the leftmost defined group in the match.
1120 The underlying behaviour of C<%+> is provided by the
1121 L<Tie::Hash::NamedCapture> module.
1123 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1124 associated with the last successful regular expression. Therefore mixing
1125 iterative access to them via C<each> may have unpredictable results.
1126 Likewise, if the last successful match changes, then the results may be
1129 This variable was added in Perl v5.10.0. The C<%{^CAPTURE}> alias was
1132 This variable is read-only and dynamically-scoped.
1134 =item @LAST_MATCH_START
1137 X<@-> X<@LAST_MATCH_START>
1139 C<$-[0]> is the offset of the start of the last successful match.
1140 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1141 I<n>-th subpattern, or undef if the subpattern did not match.
1143 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1144 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1145 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1146 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1147 last matched subgroup in the last successful match. Contrast with
1148 C<$#+>, the number of subgroups in the regular expression. Compare
1151 This array holds the offsets of the beginnings of the last
1152 successful submatches in the currently active dynamic scope.
1153 C<$-[0]> is the offset into the string of the beginning of the
1154 entire match. The I<n>th element of this array holds the offset
1155 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1156 begins, C<$-[2]> the offset where C<$2> begins, and so on.
1158 After a match against some variable C<$var>:
1162 =item C<$`> is the same as C<substr($var, 0, $-[0])>
1164 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1166 =item C<$'> is the same as C<substr($var, $+[0])>
1168 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1170 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1172 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1176 This variable was added in Perl v5.6.0.
1178 =item %{^CAPTURE_ALL}
1184 Similar to C<%+>, this variable allows access to the named capture groups
1185 in the last successful match in the currently active dynamic scope. To
1186 each capture group name found in the regular expression, it associates a
1187 reference to an array containing the list of values captured by all
1188 buffers with that name (should there be several of them), in the order
1193 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1194 foreach my $bufname (sort keys %-) {
1195 my $ary = $-{$bufname};
1196 foreach my $idx (0..$#$ary) {
1197 print "\$-{$bufname}[$idx] : ",
1198 (defined($ary->[$idx])
1213 The keys of the C<%-> hash correspond to all buffer names found in
1214 the regular expression.
1216 The behaviour of C<%-> is implemented via the
1217 L<Tie::Hash::NamedCapture> module.
1219 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1220 associated with the last successful regular expression. Therefore mixing
1221 iterative access to them via C<each> may have unpredictable results.
1222 Likewise, if the last successful match changes, then the results may be
1225 This variable was added in Perl v5.10.0. The C<%{^CAPTURE_ALL}> alias was
1228 This variable is read-only and dynamically-scoped.
1230 =item $LAST_REGEXP_CODE_RESULT
1233 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1235 The result of evaluation of the last successful C<(?{ code })>
1236 regular expression assertion (see L<perlre>). May be written to.
1238 This variable was added in Perl 5.005.
1240 =item ${^RE_DEBUG_FLAGS}
1241 X<${^RE_DEBUG_FLAGS}>
1243 The current value of the regex debugging flags. Set to 0 for no debug output
1244 even when the C<re 'debug'> module is loaded. See L<re> for details.
1246 This variable was added in Perl v5.10.0.
1248 =item ${^RE_TRIE_MAXBUF}
1249 X<${^RE_TRIE_MAXBUF}>
1251 Controls how certain regex optimisations are applied and how much memory they
1252 utilize. This value by default is 65536 which corresponds to a 512kB
1253 temporary cache. Set this to a higher value to trade
1254 memory for speed when matching large alternations. Set
1255 it to a lower value if you want the optimisations to
1256 be as conservative of memory as possible but still occur, and set it to a
1257 negative value to prevent the optimisation and conserve the most memory.
1258 Under normal situations this variable should be of no interest to you.
1260 This variable was added in Perl v5.10.0.
1264 =head2 Variables related to filehandles
1266 Variables that depend on the currently selected filehandle may be set
1267 by calling an appropriate object method on the C<IO::Handle> object,
1268 although this is less efficient than using the regular built-in
1269 variables. (Summary lines below for this contain the word HANDLE.)
1274 after which you may use either
1280 HANDLE->method(EXPR)
1282 Each method returns the old value of the C<IO::Handle> attribute. The
1283 methods each take an optional EXPR, which, if supplied, specifies the
1284 new value for the C<IO::Handle> attribute in question. If not
1285 supplied, most methods do nothing to the current value--except for
1286 C<autoflush()>, which will assume a 1 for you, just to be different.
1288 Because loading in the C<IO::Handle> class is an expensive operation,
1289 you should learn how to use the regular built-in variables.
1291 A few of these variables are considered "read-only". This means that
1292 if you try to assign to this variable, either directly or indirectly
1293 through a reference, you'll raise a run-time exception.
1295 You should be very careful when modifying the default values of most
1296 special variables described in this document. In most cases you want
1297 to localize these variables before changing them, since if you don't,
1298 the change may affect other modules which rely on the default values
1299 of the special variables that you have changed. This is one of the
1300 correct ways to read the whole file at once:
1302 open my $fh, "<", "foo" or die $!;
1303 local $/; # enable localized slurp mode
1304 my $content = <$fh>;
1307 But the following code is quite bad:
1309 open my $fh, "<", "foo" or die $!;
1310 undef $/; # enable slurp mode
1311 my $content = <$fh>;
1314 since some other module, may want to read data from some file in the
1315 default "line mode", so if the code we have just presented has been
1316 executed, the global value of C<$/> is now changed for any other code
1317 running inside the same Perl interpreter.
1319 Usually when a variable is localized you want to make sure that this
1320 change affects the shortest scope possible. So unless you are already
1321 inside some short C<{}> block, you should create one yourself. For
1325 open my $fh, "<", "foo" or die $!;
1332 Here is an example of how your own code can go broken:
1342 # do something with $_
1345 You probably expect this code to print the equivalent of
1349 but instead you get:
1353 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1354 first. The value you set in C<nasty_break()> is still there when you
1355 return. The fix is to add C<local()> so the value doesn't leak out of
1360 It's easy to notice the problem in such a short example, but in more
1361 complicated code you are looking for trouble if you don't localize
1362 changes to the special variables.
1369 Contains the name of the current file when reading from C<< <> >>.
1374 The array C<@ARGV> contains the command-line arguments intended for
1375 the script. C<$#ARGV> is generally the number of arguments minus
1376 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1377 command name itself. See L</$0> for the command name.
1382 The special filehandle that iterates over command-line filenames in
1383 C<@ARGV>. Usually written as the null filehandle in the angle operator
1384 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1385 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1386 corresponding to the last file opened by C<< <> >>. In particular,
1387 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1388 may not cause your function to automatically read the contents of all the
1394 The special filehandle that points to the currently open output file
1395 when doing edit-in-place processing with B<-i>. Useful when you have
1396 to do a lot of inserting and don't want to keep modifying C<$_>. See
1397 L<perlrun> for the B<-i> switch.
1399 =item IO::Handle->output_field_separator( EXPR )
1401 =item $OUTPUT_FIELD_SEPARATOR
1406 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1408 The output field separator for the print operator. If defined, this
1409 value is printed between each of print's arguments. Default is C<undef>.
1411 You cannot call C<output_field_separator()> on a handle, only as a
1412 static method. See L<IO::Handle|IO::Handle>.
1414 Mnemonic: what is printed when there is a "," in your print statement.
1416 =item HANDLE->input_line_number( EXPR )
1418 =item $INPUT_LINE_NUMBER
1423 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1425 Current line number for the last filehandle accessed.
1427 Each filehandle in Perl counts the number of lines that have been read
1428 from it. (Depending on the value of C<$/>, Perl's idea of what
1429 constitutes a line may not match yours.) When a line is read from a
1430 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1431 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1432 for that filehandle.
1434 You can adjust the counter by assigning to C<$.>, but this will not
1435 actually move the seek pointer. I<Localizing C<$.> will not localize
1436 the filehandle's line count>. Instead, it will localize perl's notion
1437 of which filehandle C<$.> is currently aliased to.
1439 C<$.> is reset when the filehandle is closed, but B<not> when an open
1440 filehandle is reopened without an intervening C<close()>. For more
1441 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1442 an explicit close, line numbers increase across C<ARGV> files (but see
1443 examples in L<perlfunc/eof>).
1445 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1446 line counter for a given filehandle without having to worry about
1447 which handle you last accessed.
1449 Mnemonic: many programs use "." to mean the current line number.
1451 =item IO::Handle->input_record_separator( EXPR )
1453 =item $INPUT_RECORD_SEPARATOR
1458 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1460 The input record separator, newline by default. This influences Perl's
1461 idea of what a "line" is. Works like B<awk>'s RS variable, including
1462 treating empty lines as a terminator if set to the null string (an
1463 empty line cannot contain any spaces or tabs). You may set it to a
1464 multi-character string to match a multi-character terminator, or to
1465 C<undef> to read through the end of file. Setting it to C<"\n\n">
1466 means something slightly different than setting to C<"">, if the file
1467 contains consecutive empty lines. Setting to C<""> will treat two or
1468 more consecutive empty lines as a single empty line. Setting to
1469 C<"\n\n"> will blindly assume that the next input character belongs to
1470 the next paragraph, even if it's a newline.
1472 local $/; # enable "slurp" mode
1473 local $_ = <FH>; # whole file now here
1476 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1477 be better for something. :-)
1479 Setting C<$/> to a reference to an integer, scalar containing an
1480 integer, or scalar that's convertible to an integer will attempt to
1481 read records instead of lines, with the maximum record size being the
1482 referenced integer number of characters. So this:
1484 local $/ = \32768; # or \"32768", or \$var_containing_32768
1485 open my $fh, "<", $myfile or die $!;
1488 will read a record of no more than 32768 characters from $fh. If you're
1489 not reading from a record-oriented file (or your OS doesn't have
1490 record-oriented files), then you'll likely get a full chunk of data
1491 with every read. If a record is larger than the record size you've
1492 set, you'll get the record back in pieces. Trying to set the record
1493 size to zero or less is deprecated and will cause $/ to have the value
1494 of "undef", which will cause reading in the (rest of the) whole file.
1496 As of 5.19.9 setting C<$/> to any other form of reference will throw a
1497 fatal exception. This is in preparation for supporting new ways to set
1498 C<$/> in the future.
1500 On VMS only, record reads bypass PerlIO layers and any associated
1501 buffering, so you must not mix record and non-record reads on the
1502 same filehandle. Record mode mixes with line mode only when the
1503 same buffering layer is in use for both modes.
1505 You cannot call C<input_record_separator()> on a handle, only as a
1506 static method. See L<IO::Handle|IO::Handle>.
1508 See also L<perlport/"Newlines">. Also see L</$.>.
1510 Mnemonic: / delimits line boundaries when quoting poetry.
1512 =item IO::Handle->output_record_separator( EXPR )
1514 =item $OUTPUT_RECORD_SEPARATOR
1519 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1521 The output record separator for the print operator. If defined, this
1522 value is printed after the last of print's arguments. Default is C<undef>.
1524 You cannot call C<output_record_separator()> on a handle, only as a
1525 static method. See L<IO::Handle|IO::Handle>.
1527 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1528 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1530 =item HANDLE->autoflush( EXPR )
1532 =item $OUTPUT_AUTOFLUSH
1535 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1537 If set to nonzero, forces a flush right away and after every write or
1538 print on the currently selected output channel. Default is 0
1539 (regardless of whether the channel is really buffered by the system or
1540 not; C<$|> tells you only whether you've asked Perl explicitly to
1541 flush after each write). STDOUT will typically be line buffered if
1542 output is to the terminal and block buffered otherwise. Setting this
1543 variable is useful primarily when you are outputting to a pipe or
1544 socket, such as when you are running a Perl program under B<rsh> and
1545 want to see the output as it's happening. This has no effect on input
1546 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1547 how to select the output channel. See also L<IO::Handle>.
1549 Mnemonic: when you want your pipes to be piping hot.
1554 This read-only variable contains a reference to the last-read filehandle.
1555 This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1556 This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1557 use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1558 an error or warning message.
1560 This variable was added in Perl v5.18.0.
1564 =head3 Variables related to formats
1566 The special variables for formats are a subset of those for
1567 filehandles. See L<perlform> for more information about Perl's
1575 X<$^A> X<$ACCUMULATOR>
1577 The current value of the C<write()> accumulator for C<format()> lines.
1578 A format contains C<formline()> calls that put their result into
1579 C<$^A>. After calling its format, C<write()> prints out the contents
1580 of C<$^A> and empties. So you never really see the contents of C<$^A>
1581 unless you call C<formline()> yourself and then look at it. See
1582 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1584 =item IO::Handle->format_formfeed(EXPR)
1586 =item $FORMAT_FORMFEED
1589 X<$^L> X<$FORMAT_FORMFEED>
1591 What formats output as a form feed. The default is C<\f>.
1593 You cannot call C<format_formfeed()> on a handle, only as a static
1594 method. See L<IO::Handle|IO::Handle>.
1596 =item HANDLE->format_page_number(EXPR)
1598 =item $FORMAT_PAGE_NUMBER
1601 X<$%> X<$FORMAT_PAGE_NUMBER>
1603 The current page number of the currently selected output channel.
1605 Mnemonic: C<%> is page number in B<nroff>.
1607 =item HANDLE->format_lines_left(EXPR)
1609 =item $FORMAT_LINES_LEFT
1612 X<$-> X<$FORMAT_LINES_LEFT>
1614 The number of lines left on the page of the currently selected output
1617 Mnemonic: lines_on_page - lines_printed.
1619 =item IO::Handle->format_line_break_characters EXPR
1621 =item $FORMAT_LINE_BREAK_CHARACTERS
1624 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1626 The current set of characters after which a string may be broken to
1627 fill continuation fields (starting with C<^>) in a format. The default is
1628 S<" \n-">, to break on a space, newline, or a hyphen.
1630 You cannot call C<format_line_break_characters()> on a handle, only as
1631 a static method. See L<IO::Handle|IO::Handle>.
1633 Mnemonic: a "colon" in poetry is a part of a line.
1635 =item HANDLE->format_lines_per_page(EXPR)
1637 =item $FORMAT_LINES_PER_PAGE
1640 X<$=> X<$FORMAT_LINES_PER_PAGE>
1642 The current page length (printable lines) of the currently selected
1643 output channel. The default is 60.
1645 Mnemonic: = has horizontal lines.
1647 =item HANDLE->format_top_name(EXPR)
1649 =item $FORMAT_TOP_NAME
1652 X<$^> X<$FORMAT_TOP_NAME>
1654 The name of the current top-of-page format for the currently selected
1655 output channel. The default is the name of the filehandle with C<_TOP>
1656 appended. For example, the default format top name for the C<STDOUT>
1657 filehandle is C<STDOUT_TOP>.
1659 Mnemonic: points to top of page.
1661 =item HANDLE->format_name(EXPR)
1666 X<$~> X<$FORMAT_NAME>
1668 The name of the current report format for the currently selected
1669 output channel. The default format name is the same as the filehandle
1670 name. For example, the default format name for the C<STDOUT>
1671 filehandle is just C<STDOUT>.
1673 Mnemonic: brother to C<$^>.
1677 =head2 Error Variables
1678 X<error> X<exception>
1680 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1681 about different types of error conditions that may appear during
1682 execution of a Perl program. The variables are shown ordered by
1683 the "distance" between the subsystem which reported the error and
1684 the Perl process. They correspond to errors detected by the Perl
1685 interpreter, C library, operating system, or an external program,
1688 To illustrate the differences between these variables, consider the
1689 following Perl expression, which uses a single-quoted string. After
1690 execution of this statement, perl may have set all four special error
1694 open my $pipe, "/cdrom/install |" or die $!;
1696 close $pipe or die "bad pipe: $?, $!";
1699 When perl executes the C<eval()> expression, it translates the
1700 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1701 and thence to the operating system kernel. perl sets C<$!> to
1702 the C library's C<errno> if one of these calls fails.
1704 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1705 happen if C<open> or C<close> were imported with bad prototypes), or
1706 if Perl code executed during evaluation C<die()>d. In these cases the
1707 value of C<$@> is the compile error, or the argument to C<die> (which
1708 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1710 Under a few operating systems, C<$^E> may contain a more verbose error
1711 indicator, such as in this case, "CDROM tray not closed." Systems that
1712 do not support extended error messages leave C<$^E> the same as C<$!>.
1714 Finally, C<$?> may be set to a non-0 value if the external program
1715 F</cdrom/install> fails. The upper eight bits reflect specific error
1716 conditions encountered by the program (the program's C<exit()> value).
1717 The lower eight bits reflect mode of failure, like signal death and
1718 core dump information. See L<wait(2)> for details. In contrast to
1719 C<$!> and C<$^E>, which are set only if an error condition is detected,
1720 the variable C<$?> is set on each C<wait> or pipe C<close>,
1721 overwriting the old value. This is more like C<$@>, which on every
1722 C<eval()> is always set on failure and cleared on success.
1724 For more details, see the individual descriptions at C<$@>, C<$!>,
1729 =item ${^CHILD_ERROR_NATIVE}
1730 X<$^CHILD_ERROR_NATIVE>
1732 The native status returned by the last pipe close, backtick (C<``>)
1733 command, successful call to C<wait()> or C<waitpid()>, or from the
1734 C<system()> operator. On POSIX-like systems this value can be decoded
1735 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1736 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1738 Under VMS this reflects the actual VMS exit status; i.e. it is the
1739 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1741 This variable was added in Perl v5.10.0.
1743 =item $EXTENDED_OS_ERROR
1746 X<$^E> X<$EXTENDED_OS_ERROR>
1748 Error information specific to the current operating system. At the
1749 moment, this differs from C<L</$!>> under only VMS, OS/2, and Win32 (and
1750 for MacPerl). On all other platforms, C<$^E> is always just the same
1753 Under VMS, C<$^E> provides the VMS status value from the last system
1754 error. This is more specific information about the last system error
1755 than that provided by C<$!>. This is particularly important when C<$!>
1756 is set to B<EVMSERR>.
1758 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1759 API either via CRT, or directly from perl.
1761 Under Win32, C<$^E> always returns the last error information reported
1762 by the Win32 call C<GetLastError()> which describes the last error
1763 from within the Win32 API. Most Win32-specific code will report errors
1764 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1765 portable Perl code will report errors via C<$!>.
1767 Caveats mentioned in the description of C<L</$!>> generally apply to
1770 This variable was added in Perl 5.003.
1772 Mnemonic: Extra error explanation.
1774 =item $EXCEPTIONS_BEING_CAUGHT
1777 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1779 Current state of the interpreter.
1782 --------- -------------------------------------
1783 undef Parsing module, eval, or main program
1784 true (1) Executing an eval
1787 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1790 The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1791 the C<undef> value does not indicate whether exceptions are being caught,
1792 since compilation of the main program does not catch exceptions.
1794 This variable was added in Perl 5.004.
1801 The current value of the warning switch, initially true if B<-w> was
1802 used, false otherwise, but directly modifiable.
1804 See also L<warnings>.
1806 Mnemonic: related to the B<-w> switch.
1808 =item ${^WARNING_BITS}
1811 The current set of warning checks enabled by the C<use warnings> pragma.
1812 It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1813 values are considered internal to the L<warnings> pragma and may change
1814 between versions of Perl.
1816 This variable was added in Perl v5.6.0.
1823 X<$!> X<$ERRNO> X<$OS_ERROR>
1825 When referenced, C<$!> retrieves the current value
1826 of the C C<errno> integer variable.
1827 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1828 When referenced as a string, C<$!> yields the system error string
1829 corresponding to C<errno>.
1831 Many system or library calls set C<errno> if they fail,
1832 to indicate the cause of failure. They usually do B<not>
1833 set C<errno> to zero if they succeed. This means C<errno>,
1834 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1836 if (open my $fh, "<", $filename) {
1837 # Here $! is meaningless.
1841 # ONLY here is $! meaningful.
1843 # Already here $! might be meaningless.
1845 # Since here we might have either success or failure,
1846 # $! is meaningless.
1848 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1849 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1850 It can be used immediately before invoking the C<die()> operator,
1851 to set the exit value, or to inspect the system error string
1852 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1854 Mnemonic: What just went bang?
1861 X<%!> X<%OS_ERROR> X<%ERRNO>
1863 Each element of C<%!> has a true value only if C<$!> is set to that
1864 value. For example, C<$!{ENOENT}> is true if and only if the current
1865 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1866 such file or directory" (or its moral equivalent: not all operating
1867 systems give that exact error, and certainly not all languages). The
1868 specific true value is not guaranteed, but in the past has generally
1869 been the numeric value of C<$!>. To check if a particular key is
1870 meaningful on your system, use C<exists $!{the_key}>; for a list of legal
1871 keys, use C<keys %!>. See L<Errno> for more information, and also see
1874 This variable was added in Perl 5.005.
1879 X<$?> X<$CHILD_ERROR>
1881 The status returned by the last pipe close, backtick (C<``>) command,
1882 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1883 operator. This is just the 16-bit status word returned by the
1884 traditional Unix C<wait()> system call (or else is made up to look
1885 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1886 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1887 from, and C<$? & 128> reports whether there was a core dump.
1889 Additionally, if the C<h_errno> variable is supported in C, its value
1890 is returned via C<$?> if any C<gethost*()> function fails.
1892 If you have installed a signal handler for C<SIGCHLD>, the
1893 value of C<$?> will usually be wrong outside that handler.
1895 Inside an C<END> subroutine C<$?> contains the value that is going to be
1896 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1897 change the exit status of your program. For example:
1900 $? = 1 if $? == 255; # die would make it 255
1903 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1904 actual VMS exit status, instead of the default emulation of POSIX
1905 status; see L<perlvms/$?> for details.
1907 Mnemonic: similar to B<sh> and B<ksh>.
1912 X<$@> X<$EVAL_ERROR>
1914 The Perl error from the last C<eval> operator, i.e. the last exception that
1915 was caught. For C<eval BLOCK>, this is either a runtime error message or the
1916 string or reference C<die> was called with. The C<eval STRING> form also
1917 catches syntax errors and other compile time exceptions.
1919 If no error occurs, C<eval> sets C<$@> to the empty string.
1921 Warning messages are not collected in this variable. You can, however,
1922 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1923 described in L</%SIG>.
1925 Mnemonic: Where was the error "at"?
1929 =head2 Variables related to the interpreter state
1931 These variables provide information about the current interpreter state.
1938 X<$^C> X<$COMPILING>
1940 The current value of the flag associated with the B<-c> switch.
1941 Mainly of use with B<-MO=...> to allow code to alter its behavior
1942 when being compiled, such as for example to C<AUTOLOAD> at compile
1943 time rather than normal, deferred loading. Setting
1944 C<$^C = 1> is similar to calling C<B::minus_c>.
1946 This variable was added in Perl v5.6.0.
1951 X<$^D> X<$DEBUGGING>
1953 The current value of the debugging flags. May be read or set. Like its
1954 L<command-line equivalent|perlrun/B<-D>I<letters>>, you can use numeric
1955 or symbolic values, e.g. C<$^D = 10> or C<$^D = "st">. See
1956 L<perlrun/B<-D>I<number>>. The contents of this variable also affects the
1957 debugger operation. See L<perldebguts/Debugger Internals>.
1959 Mnemonic: value of B<-D> switch.
1964 This variable is no longer supported.
1966 It used to hold the I<object reference> to the C<Encode> object that was
1967 used to convert the source code to Unicode.
1969 Its purpose was to allow your non-ASCII Perl
1970 scripts not to have to be written in UTF-8; this was
1971 useful before editors that worked on UTF-8 encoded text were common, but
1972 that was long ago. It caused problems, such as affecting the operation
1973 of other modules that weren't expecting it, causing general mayhem.
1975 If you need something like this functionality, it is recommended that use
1976 you a simple source filter, such as L<Filter::Encoding>.
1978 If you are coming here because code of yours is being adversely affected
1979 by someone's use of this variable, you can usually work around it by
1984 near the beginning of the functions that are getting broken. This
1985 undefines the variable during the scope of execution of the including
1988 This variable was added in Perl 5.8.2 and removed in 5.26.0.
1989 Setting it to anything other than C<undef> was made fatal in Perl 5.28.0.
1991 =item ${^GLOBAL_PHASE}
1994 The current phase of the perl interpreter.
1996 Possible values are:
2002 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
2003 value is mostly there for completeness and for use via the
2004 underlying C variable C<PL_phase>. It's not really possible for Perl
2005 code to be executed unless construction of the interpreter is
2010 This is the global compile-time. That includes, basically, every
2011 C<BEGIN> block executed directly or indirectly from during the
2012 compile-time of the top-level program.
2014 This phase is not called "BEGIN" to avoid confusion with
2015 C<BEGIN>-blocks, as those are executed during compile-time of any
2016 compilation unit, not just the top-level program. A new, localised
2017 compile-time entered at run-time, for example by constructs as
2018 C<eval "use SomeModule"> are not global interpreter phases, and
2019 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
2023 Execution of any C<CHECK> blocks.
2027 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
2031 The main run-time, i.e. the execution of C<PL_main_root>.
2035 Execution of any C<END> blocks.
2043 Also note that there's no value for UNITCHECK-blocks. That's because
2044 those are run for each compilation unit individually, and therefore is
2045 not a global interpreter phase.
2047 Not every program has to go through each of the possible phases, but
2048 transition from one phase to another can only happen in the order
2049 described in the above list.
2051 An example of all of the phases Perl code can see:
2053 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
2055 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
2057 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
2060 package Print::Phase;
2063 my ($class, $time) = @_;
2064 return bless \$time, $class;
2069 print "$$self: ${^GLOBAL_PHASE}\n";
2073 print "run-time: ${^GLOBAL_PHASE}\n";
2075 my $runtime = Print::Phase->new(
2076 "lexical variables are garbage collected before END"
2079 END { print "end-time: ${^GLOBAL_PHASE}\n" }
2081 our $destruct = Print::Phase->new(
2082 "package variables are garbage collected after END"
2091 lexical variables are garbage collected before END: RUN
2093 package variables are garbage collected after END: DESTRUCT
2095 This variable was added in Perl 5.14.0.
2100 WARNING: This variable is strictly for
2101 internal use only. Its availability,
2102 behavior, and contents are subject to change without notice.
2104 This variable contains compile-time hints for the Perl interpreter. At the
2105 end of compilation of a BLOCK the value of this variable is restored to the
2106 value when the interpreter started to compile the BLOCK.
2108 When perl begins to parse any block construct that provides a lexical scope
2109 (e.g., eval body, required file, subroutine body, loop body, or conditional
2110 block), the existing value of C<$^H> is saved, but its value is left unchanged.
2111 When the compilation of the block is completed, it regains the saved value.
2112 Between the points where its value is saved and restored, code that
2113 executes within BEGIN blocks is free to change the value of C<$^H>.
2115 This behavior provides the semantic of lexical scoping, and is used in,
2116 for instance, the C<use strict> pragma.
2118 The contents should be an integer; different bits of it are used for
2119 different pragmatic flags. Here's an example:
2121 sub add_100 { $^H |= 0x100 }
2128 Consider what happens during execution of the BEGIN block. At this point
2129 the BEGIN block has already been compiled, but the body of C<foo()> is still
2130 being compiled. The new value of C<$^H>
2131 will therefore be visible only while
2132 the body of C<foo()> is being compiled.
2134 Substitution of C<BEGIN { add_100() }> block with:
2136 BEGIN { require strict; strict->import('vars') }
2138 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
2139 version of the same lexical pragma:
2142 require strict; strict->import('vars') if $condition
2145 This variable was added in Perl 5.003.
2150 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2151 it useful for implementation of lexically scoped pragmas. See
2152 L<perlpragma>. All the entries are stringified when accessed at
2153 runtime, so only simple values can be accommodated. This means no
2154 pointers to objects, for example.
2156 When putting items into C<%^H>, in order to avoid conflicting with other
2157 users of the hash there is a convention regarding which keys to use.
2158 A module should use only keys that begin with the module's name (the
2159 name of its main package) and a "/" character. For example, a module
2160 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2162 This variable was added in Perl v5.6.0.
2167 An internal variable used by PerlIO. A string in two parts, separated
2168 by a C<\0> byte, the first part describes the input layers, the second
2169 part describes the output layers.
2171 This variable was added in Perl v5.8.0.
2178 The internal variable for debugging support. The meanings of the
2179 various bits are subject to change, but currently indicate:
2185 Debug subroutine enter/exit.
2189 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2190 each statement executed. Also causes saving source code lines (like
2195 Switch off optimizations.
2199 Preserve more data for future interactive inspections.
2203 Keep info about source lines on which a subroutine is defined.
2207 Start with single-step on.
2211 Use subroutine address instead of name when reporting.
2215 Report C<goto &subroutine> as well.
2219 Provide informative "file" names for evals based on the place they were compiled.
2223 Provide informative names to anonymous subroutines based on the place they
2228 Save source code lines into C<@{"_<$filename"}>.
2232 When saving source, include evals that generate no subroutines.
2236 When saving source, include source that did not compile.
2240 Some bits may be relevant at compile-time only, some at
2241 run-time only. This is a new mechanism and the details may change.
2242 See also L<perldebguts>.
2247 Reflects if taint mode is on or off. 1 for on (the program was run with
2248 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2251 This variable is read-only.
2253 This variable was added in Perl v5.8.0.
2255 =item ${^SAFE_LOCALES}
2258 Reflects if safe locale operations are available to this perl (when the
2259 value is 1) or not (the value is 0). This variable is always 1 if the
2260 perl has been compiled without threads. It is also 1 if this perl is
2261 using thread-safe locale operations. Note that an individual thread may
2262 choose to use the global locale (generally unsafe) by calling
2263 L<perlapi/switch_to_global_locale>. This variable currently is still
2264 set to 1 in such threads.
2266 This variable is read-only.
2268 This variable was added in Perl v5.28.0.
2273 Reflects certain Unicode settings of Perl. See L<perlrun>
2274 documentation for the C<-C> switch for more information about
2275 the possible values.
2277 This variable is set during Perl startup and is thereafter read-only.
2279 This variable was added in Perl v5.8.2.
2284 This variable controls the state of the internal UTF-8 offset caching code.
2285 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2286 all its results against linear scans, and panicking on any discrepancy.
2288 This variable was added in Perl v5.8.9. It is subject to change or
2289 removal without notice, but is currently used to avoid recalculating the
2290 boundaries of multi-byte UTF-8-encoded characters.
2292 =item ${^UTF8LOCALE}
2295 This variable indicates whether a UTF-8 locale was detected by perl at
2296 startup. This information is used by perl when it's in
2297 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2298 switch); see L<perlrun> for more info on this.
2300 This variable was added in Perl v5.8.8.
2304 =head2 Deprecated and removed variables
2306 Deprecating a variable announces the intent of the perl maintainers to
2307 eventually remove the variable from the language. It may still be
2308 available despite its status. Using a deprecated variable triggers
2311 Once a variable is removed, its use triggers an error telling you
2312 the variable is unsupported.
2314 See L<perldiag> for details about error messages.
2321 C<$#> was a variable that could be used to format printed numbers.
2322 After a deprecation cycle, its magic was removed in Perl v5.10.0 and
2323 using it now triggers a warning: C<$# is no longer supported>.
2325 This is not the sigil you use in front of an array name to get the
2326 last index, like C<$#array>. That's still how you get the last index
2327 of an array in Perl. The two have nothing to do with each other.
2329 Deprecated in Perl 5.
2331 Removed in Perl v5.10.0.
2336 C<$*> was a variable that you could use to enable multiline matching.
2337 After a deprecation cycle, its magic was removed in Perl v5.10.0.
2338 Using it now triggers a warning: C<$* is no longer supported>.
2339 You should use the C</s> and C</m> regexp modifiers instead.
2341 Deprecated in Perl 5.
2343 Removed in Perl v5.10.0.
2348 This variable stores the index of the first element in an array, and
2349 of the first character in a substring. The default is 0, but you could
2350 theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2351 when subscripting and when evaluating the index() and substr() functions.
2353 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2354 directive, and cannot influence the behavior of any other file.
2355 (That's why you can only assign compile-time constants to it.)
2356 Its use is highly discouraged.
2358 Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
2359 scopes in the same file, unlike other compile-time directives (such as
2360 L<strict>). Using local() on it would bind its value strictly to a lexical
2361 block. Now it is always lexically scoped.
2363 As of Perl v5.16.0, it is implemented by the L<arybase> module. See
2364 L<arybase> for more details on its behaviour.
2366 Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2367 effect, and always contains 0. Assigning 0 to it is permitted, but any
2368 other value will produce an error.
2370 Mnemonic: [ begins subscripts.
2372 Deprecated in Perl v5.12.0.
2374 Assigning a non-zero value be fatal in Perl v5.30.0.