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Add thread-safe locale handling
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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
b0c22438 7=head2 The Syntax of Variable Names
8
241a59d9 9Variable names in Perl can have several formats. Usually, they
b0c22438 10must begin with a letter or underscore, in which case they can be
11arbitrarily long (up to an internal limit of 251 characters) and
12may contain letters, digits, underscores, or the special sequence
241a59d9 13C<::> or C<'>. In this case, the part before the last C<::> or
b0c22438 14C<'> is taken to be a I<package qualifier>; see L<perlmod>.
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15A Unicode letter that is not ASCII is not considered to be a letter
16unless S<C<"use utf8">> is in effect, and somewhat more complicated
17rules apply; see L<perldata/Identifier parsing> for details.
b0c22438 18
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19Perl variable names may also be a sequence of digits, a single
20punctuation character, or the two-character sequence: C<^> (caret or
21CIRCUMFLEX ACCENT) followed by any one of the characters C<[][A-Z^_?\]>.
22These names are all reserved for
b0c22438 23special uses by Perl; for example, the all-digits names are used
24to hold data captured by backreferences after a regular expression
ce4793f1 25match.
b0c22438 26
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27Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
28preceded by a caret. These must all be written in the form C<${^Foo}>;
29the braces are not optional. C<${^Foo}> denotes the scalar variable
30whose name is considered to be a control-C<F> followed by two C<o>'s.
31These variables are
b0c22438 32reserved for future special uses by Perl, except for the ones that
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33begin with C<^_> (caret-underscore). No
34name that begins with C<^_> will acquire a special
b0c22438 35meaning in any future version of Perl; such names may therefore be
241a59d9 36used safely in programs. C<$^_> itself, however, I<is> reserved.
b0c22438 37
ce4793f1 38Perl identifiers that begin with digits or
b0c22438 39punctuation characters are exempt from the effects of the C<package>
40declaration and are always forced to be in package C<main>; they are
241a59d9 41also exempt from C<strict 'vars'> errors. A few other names are also
b0c22438 42exempt in these ways:
43
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44 ENV STDIN
45 INC STDOUT
46 ARGV STDERR
47 ARGVOUT
48 SIG
b0c22438 49
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
53
54=head1 SPECIAL VARIABLES
a0d0e21e 55
241a59d9 56The following names have special meaning to Perl. Most punctuation
0b9346e6 57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
9548c15c 60 use English;
a0d0e21e 61
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62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
1e7d0944 64borrowed from B<awk>. For more info, please see L<English>.
a1ce9542 65
241a59d9 66Before you continue, note the sort order for variables. In general, we
0b9346e6 67first list the variables in case-insensitive, almost-lexigraphical
68order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
69or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
70For variables with the same identifier, we list it in order of scalar,
71array, hash, and bareword.
a1ce9542 72
b0c22438 73=head2 General Variables
a0d0e21e 74
84dabc03 75=over 8
76
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77=item $ARG
78
79=item $_
a054c801 80X<$_> X<$ARG>
a0d0e21e 81
241a59d9 82The default input and pattern-searching space. The following pairs are
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83equivalent:
84
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85 while (<>) {...} # equivalent only in while!
86 while (defined($_ = <>)) {...}
a0d0e21e 87
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88 /^Subject:/
89 $_ =~ /^Subject:/
a0d0e21e 90
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91 tr/a-z/A-Z/
92 $_ =~ tr/a-z/A-Z/
a0d0e21e 93
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94 chomp
95 chomp($_)
a0d0e21e 96
0b9346e6 97Here are the places where Perl will assume C<$_> even if you don't use it:
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98
99=over 3
100
101=item *
102
84dabc03 103The following functions use C<$_> as a default argument:
db1511c8 104
f61f53cc 105abs, alarm, chomp, chop, chr, chroot,
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106cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
107lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
b0169937 108quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
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109rmdir, say, sin, split (for its second
110argument), sqrt, stat, study, uc, ucfirst,
b0169937 111unlink, unpack.
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112
113=item *
114
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115All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
116See L<perlfunc/-X>
117
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118=item *
119
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120The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
121when used without an C<=~> operator.
cb1a09d0 122
54310121 123=item *
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124
125The default iterator variable in a C<foreach> loop if no other
126variable is supplied.
127
54310121 128=item *
cb1a09d0 129
b0c22438 130The implicit iterator variable in the C<grep()> and C<map()> functions.
cb1a09d0 131
54310121 132=item *
cb1a09d0 133
b0c22438 134The implicit variable of C<given()>.
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135
136=item *
137
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138The default place to put the next value or input record
139when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
cb1a09d0 140operation's result is tested by itself as the sole criterion of a C<while>
241a59d9 141test. Outside a C<while> test, this will not happen.
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142
143=back
144
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145C<$_> is by default a global variable. However, as
146of perl v5.10.0, you can use a lexical version of
241a59d9 147C<$_> by declaring it in a file or in a block with C<my>. Moreover,
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148declaring C<our $_> restores the global C<$_> in the current scope. Though
149this seemed like a good idea at the time it was introduced, lexical C<$_>
150actually causes more problems than it solves. If you call a function that
151expects to be passed information via C<$_>, it may or may not work,
152depending on how the function is written, there not being any easy way to
153solve this. Just avoid lexical C<$_>, unless you are feeling particularly
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154masochistic. For this reason lexical C<$_> is still experimental and will
155produce a warning unless warnings have been disabled. As with other
156experimental features, the behavior of lexical C<$_> is subject to change
157without notice, including change into a fatal error.
59f00321 158
b0c22438 159Mnemonic: underline is understood in certain operations.
a0d0e21e 160
0b9346e6 161=item @ARG
cde0cee5 162
0b9346e6 163=item @_
164X<@_> X<@ARG>
a0d0e21e 165
0b9346e6 166Within a subroutine the array C<@_> contains the parameters passed to
241a59d9 167that subroutine. Inside a subroutine, C<@_> is the default array for
256ca3d3 168the array operators C<pop> and C<shift>.
a0d0e21e 169
0b9346e6 170See L<perlsub>.
a0d0e21e 171
1311257d 172=item $LIST_SEPARATOR
173
174=item $"
175X<$"> X<$LIST_SEPARATOR>
176
69520822 177When an array or an array slice is interpolated into a double-quoted
178string or a similar context such as C</.../>, its elements are
241a59d9 179separated by this value. Default is a space. For example, this:
69520822 180
9548c15c 181 print "The array is: @array\n";
69520822 182
183is equivalent to this:
184
9548c15c 185 print "The array is: " . join($", @array) . "\n";
69520822 186
187Mnemonic: works in double-quoted context.
1311257d 188
b0c22438 189=item $PROCESS_ID
cde0cee5 190
b0c22438 191=item $PID
a0d0e21e 192
b0c22438 193=item $$
194X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 195
241a59d9 196The process number of the Perl running this script. Though you I<can> set
4a904372 197this variable, doing so is generally discouraged, although it can be
241a59d9 198invaluable for some testing purposes. It will be reset automatically
b0c22438 199across C<fork()> calls.
a0d0e21e 200
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201Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
202would emulate POSIX semantics on Linux systems using LinuxThreads, a
203partial implementation of POSIX Threads that has since been superseded
204by the Native POSIX Thread Library (NPTL).
205
e3f68f70 206LinuxThreads is now obsolete on Linux, and caching C<getpid()>
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207like this made embedding perl unnecessarily complex (since you'd have
208to manually update the value of $$), so now C<$$> and C<getppid()>
209will always return the same values as the underlying C library.
210
211Debian GNU/kFreeBSD systems also used LinuxThreads up until and
212including the 6.0 release, but after that moved to FreeBSD thread
213semantics, which are POSIX-like.
214
215To see if your system is affected by this discrepancy check if
216C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
1e7d0944 217value. NTPL threads preserve the POSIX semantics.
a0d0e21e 218
b0c22438 219Mnemonic: same as shells.
ad83b128 220
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221=item $PROGRAM_NAME
222
223=item $0
224X<$0> X<$PROGRAM_NAME>
225
226Contains the name of the program being executed.
227
228On some (but not all) operating systems assigning to C<$0> modifies
241a59d9 229the argument area that the C<ps> program sees. On some platforms you
66d7055b 230may have to use special C<ps> options or a different C<ps> to see the
241a59d9 231changes. Modifying the C<$0> is more useful as a way of indicating the
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232current program state than it is for hiding the program you're
233running.
234
235Note that there are platform-specific limitations on the maximum
241a59d9 236length of C<$0>. In the most extreme case it may be limited to the
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237space occupied by the original C<$0>.
238
239In some platforms there may be arbitrary amount of padding, for
240example space characters, after the modified name as shown by C<ps>.
241In some platforms this padding may extend all the way to the original
242length of the argument area, no matter what you do (this is the case
243for example with Linux 2.2).
244
245Note for BSD users: setting C<$0> does not completely remove "perl"
241a59d9 246from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
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247result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
248and the " (perl)" suffix are shown depends on your exact BSD variant
241a59d9 249and version). This is an operating system feature, Perl cannot help it.
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250
251In multithreaded scripts Perl coordinates the threads so that any
252thread may modify its copy of the C<$0> and the change becomes visible
241a59d9 253to ps(1) (assuming the operating system plays along). Note that
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254the view of C<$0> the other threads have will not change since they
255have their own copies of it.
256
257If the program has been given to perl via the switches C<-e> or C<-E>,
258C<$0> will contain the string C<"-e">.
259
60cf4914 260On Linux as of perl v5.14.0 the legacy process name will be set with
66d7055b 261C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
241a59d9 262perl has done since version 4.000. Now system utilities that read the
66d7055b 263legacy process name such as ps, top and killall will recognize the
241a59d9 264name you set when assigning to C<$0>. The string you supply will be
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265cut off at 16 bytes, this is a limitation imposed by Linux.
266
267Mnemonic: same as B<sh> and B<ksh>.
268
b0c22438 269=item $REAL_GROUP_ID
a01268b5 270
b0c22438 271=item $GID
a01268b5 272
b0c22438 273=item $(
274X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 275
241a59d9 276The real gid of this process. If you are on a machine that supports
b0c22438 277membership in multiple groups simultaneously, gives a space separated
241a59d9 278list of groups you are in. The first number is the one returned by
b0c22438 279C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
280the same as the first number.
a01268b5 281
b0c22438 282However, a value assigned to C<$(> must be a single number used to
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283set the real gid. So the value given by C<$(> should I<not> be assigned
284back to C<$(> without being forced numeric, such as by adding zero. Note
b0c22438 285that this is different to the effective gid (C<$)>) which does take a
286list.
fe307981 287
b0c22438 288You can change both the real gid and the effective gid at the same
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289time by using C<POSIX::setgid()>. Changes
290to C<$(> require a check to C<$!>
b0c22438 291to detect any possible errors after an attempted change.
6cef1e77 292
241a59d9 293Mnemonic: parentheses are used to I<group> things. The real gid is the
b0c22438 294group you I<left>, if you're running setgid.
6cef1e77 295
b0c22438 296=item $EFFECTIVE_GROUP_ID
8e08999f 297
b0c22438 298=item $EGID
81714fb9 299
b0c22438 300=item $)
301X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
81714fb9 302
241a59d9 303The effective gid of this process. If you are on a machine that
b0c22438 304supports membership in multiple groups simultaneously, gives a space
241a59d9 305separated list of groups you are in. The first number is the one
b0c22438 306returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
307one of which may be the same as the first number.
81714fb9 308
b0c22438 309Similarly, a value assigned to C<$)> must also be a space-separated
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310list of numbers. The first number sets the effective gid, and
311the rest (if any) are passed to C<setgroups()>. To get the effect of an
b0c22438 312empty list for C<setgroups()>, just repeat the new effective gid; that is,
313to force an effective gid of 5 and an effectively empty C<setgroups()>
314list, say C< $) = "5 5" >.
81714fb9 315
b0c22438 316You can change both the effective gid and the real gid at the same
317time by using C<POSIX::setgid()> (use only a single numeric argument).
318Changes to C<$)> require a check to C<$!> to detect any possible errors
319after an attempted change.
44a2ac75 320
b0c22438 321C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
241a59d9 322machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
b0c22438 323and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 324
241a59d9 325Mnemonic: parentheses are used to I<group> things. The effective gid
b0c22438 326is the group that's I<right> for you, if you're running setgid.
44a2ac75 327
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328=item $REAL_USER_ID
329
330=item $UID
331
332=item $<
333X<< $< >> X<$UID> X<$REAL_USER_ID>
334
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335The real uid of this process. You can change both the real uid and the
336effective uid at the same time by using C<POSIX::setuid()>. Since
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337changes to C<< $< >> require a system call, check C<$!> after a change
338attempt to detect any possible errors.
339
340Mnemonic: it's the uid you came I<from>, if you're running setuid.
341
342=item $EFFECTIVE_USER_ID
343
344=item $EUID
345
346=item $>
347X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
348
241a59d9 349The effective uid of this process. For example:
c82f2f4e 350
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351 $< = $>; # set real to effective uid
352 ($<,$>) = ($>,$<); # swap real and effective uids
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353
354You can change both the effective uid and the real uid at the same
241a59d9 355time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
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356to C<$!> to detect any possible errors after an attempted change.
357
358C<< $< >> and C<< $> >> can be swapped only on machines
359supporting C<setreuid()>.
360
361Mnemonic: it's the uid you went I<to>, if you're running setuid.
362
0b9346e6 363=item $SUBSCRIPT_SEPARATOR
364
365=item $SUBSEP
366
367=item $;
368X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
369
241a59d9 370The subscript separator for multidimensional array emulation. If you
0b9346e6 371refer to a hash element as
372
592708b4 373 $foo{$x,$y,$z}
0b9346e6 374
375it really means
376
592708b4 377 $foo{join($;, $x, $y, $z)}
0b9346e6 378
379But don't put
380
592708b4 381 @foo{$x,$y,$z} # a slice--note the @
0b9346e6 382
383which means
384
592708b4 385 ($foo{$x},$foo{$y},$foo{$z})
0b9346e6 386
241a59d9 387Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 388binary data there might not be any safe value for C<$;>.
389
390Consider using "real" multidimensional arrays as described
391in L<perllol>.
392
393Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
394
0b9346e6 395=item $a
396
397=item $b
398X<$a> X<$b>
399
400Special package variables when using C<sort()>, see L<perlfunc/sort>.
401Because of this specialness C<$a> and C<$b> don't need to be declared
402(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
241a59d9 403pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
0b9346e6 404be able to use them in the C<sort()> comparison block or function.
405
0b9346e6 406=item %ENV
407X<%ENV>
408
241a59d9 409The hash C<%ENV> contains your current environment. Setting a
0b9346e6 410value in C<ENV> changes the environment for any child processes
411you subsequently C<fork()> off.
412
32e006ac 413As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
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414
415 my $foo = 1;
416 $ENV{'bar'} = \$foo;
417 if( ref $ENV{'bar'} ) {
32e006ac 418 say "Pre 5.18.0 Behaviour";
a5effbbc 419 } else {
32e006ac 420 say "Post 5.18.0 Behaviour";
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421 }
422
423Previously, only child processes received stringified values:
424
425 my $foo = 1;
426 $ENV{'bar'} = \$foo;
427
428 # Always printed 'non ref'
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429 system($^X, '-e',
430 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
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431
432This happens because you can't really share arbitrary data structures with
433foreign processes.
434
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435=item $OLD_PERL_VERSION
436
4ad0ecd4 437=item $]
b77ebf74 438X<$]> X<$OLD_PERL_VERSION>
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439
440The revision, version, and subversion of the Perl interpreter, represented
441as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
442is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
443
444This variable can be used to determine whether the Perl interpreter
445executing a script is in the right range of versions:
446
447 warn "No PerlIO!\n" if $] lt '5.008';
448
449When comparing C<$]>, string comparison operators are B<highly
450recommended>. The inherent limitations of binary floating point
451representation can sometimes lead to incorrect comparisons for some
452numbers on some architectures.
453
454See also the documentation of C<use VERSION> and C<require VERSION>
455for a convenient way to fail if the running Perl interpreter is too old.
456
457See L</$^V> for a representation of the Perl version as a L<version>
458object, which allows more flexible string comparisons.
459
460The main advantage of C<$]> over C<$^V> is that it works the same on any
461version of Perl. The disadvantages are that it can't easily be compared
462to versions in other formats (e.g. literal v-strings, "v1.2.3" or
463version objects) and numeric comparisons can occasionally fail; it's good
464for string literal version checks and bad for comparing to a variable
465that hasn't been sanity-checked.
466
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467The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
468reasons but its use is discouraged. (If your reason to use C<$]> is to
469run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
470be self-defeating.)
4ad0ecd4 471
9fcc2314 472Mnemonic: Is this version of perl in the right bracket?
83c1fffe 473
b0c22438 474=item $SYSTEM_FD_MAX
5b2b9c68 475
b0c22438 476=item $^F
477X<$^F> X<$SYSTEM_FD_MAX>
5b2b9c68 478
241a59d9 479The maximum system file descriptor, ordinarily 2. System file
b0c22438 480descriptors are passed to C<exec()>ed processes, while higher file
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481descriptors are not. Also, during an
482C<open()>, system file descriptors are
b0c22438 483preserved even if the C<open()> fails (ordinary file descriptors are
241a59d9 484closed before the C<open()> is attempted). The close-on-exec
b0c22438 485status of a file descriptor will be decided according to the value of
486C<$^F> when the corresponding file, pipe, or socket was opened, not the
487time of the C<exec()>.
5b2b9c68 488
0b9346e6 489=item @F
490X<@F>
491
492The array C<@F> contains the fields of each line read in when autosplit
241a59d9 493mode is turned on. See L<perlrun> for the B<-a> switch. This array
0b9346e6 494is package-specific, and must be declared or given a full package name
495if not in package main when running under C<strict 'vars'>.
496
0b9346e6 497=item @INC
498X<@INC>
499
500The array C<@INC> contains the list of places that the C<do EXPR>,
241a59d9 501C<require>, or C<use> constructs look for their library files. It
0b9346e6 502initially consists of the arguments to any B<-I> command-line
503switches, followed by the default Perl library, probably
504F</usr/local/lib/perl>, followed by ".", to represent the current
241a59d9 505directory. ("." will not be appended if taint checks are enabled,
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506either by C<-T> or by C<-t>, or if configured not to do so by the
507C<-Ddefault_inc_excludes_dot> compile time option.) If you need to
508modify this at runtime, you should use the C<use lib> pragma to get
509the machine-dependent library properly loaded also:
0b9346e6 510
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511 use lib '/mypath/libdir/';
512 use SomeMod;
0b9346e6 513
514You can also insert hooks into the file inclusion system by putting Perl
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515code directly into C<@INC>. Those hooks may be subroutine references,
516array references or blessed objects. See L<perlfunc/require> for details.
0b9346e6 517
518=item %INC
519X<%INC>
520
521The hash C<%INC> contains entries for each filename included via the
241a59d9 522C<do>, C<require>, or C<use> operators. The key is the filename
0b9346e6 523you specified (with module names converted to pathnames), and the
241a59d9 524value is the location of the file found. The C<require>
0b9346e6 525operator uses this hash to determine whether a particular file has
526already been included.
527
528If the file was loaded via a hook (e.g. a subroutine reference, see
529L<perlfunc/require> for a description of these hooks), this hook is
241a59d9 530by default inserted into C<%INC> in place of a filename. Note, however,
0b9346e6 531that the hook may have set the C<%INC> entry by itself to provide some more
532specific info.
533
b0c22438 534=item $INPLACE_EDIT
a0d0e21e 535
b0c22438 536=item $^I
537X<$^I> X<$INPLACE_EDIT>
a0d0e21e 538
241a59d9 539The current value of the inplace-edit extension. Use C<undef> to disable
b0c22438 540inplace editing.
a0d0e21e 541
b0c22438 542Mnemonic: value of B<-i> switch.
a0d0e21e 543
bf38d944
H
544=item @ISA
545X<@ISA>
546
547Each package contains a special array called C<@ISA> which contains a list
548of that class's parent classes, if any. This array is simply a list of
549scalars, each of which is a string that corresponds to a package name. The
550array is examined when Perl does method resolution, which is covered in
551L<perlobj>.
552
5312fe6c
AP
553To load packages while adding them to C<@ISA>, see the L<parent> pragma. The
554discouraged L<base> pragma does this as well, but should not be used except
555when compatibility with the discouraged L<fields> pragma is required.
bf38d944 556
b0c22438 557=item $^M
558X<$^M>
a0d0e21e 559
b0c22438 560By default, running out of memory is an untrappable, fatal error.
561However, if suitably built, Perl can use the contents of C<$^M>
241a59d9 562as an emergency memory pool after C<die()>ing. Suppose that your Perl
b0c22438 563were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
564Then
a0d0e21e 565
9548c15c 566 $^M = 'a' x (1 << 16);
a0d0e21e 567
241a59d9 568would allocate a 64K buffer for use in an emergency. See the
b0c22438 569F<INSTALL> file in the Perl distribution for information on how to
241a59d9 570add custom C compilation flags when compiling perl. To discourage casual
b0c22438 571use of this advanced feature, there is no L<English|English> long name for
572this variable.
a0d0e21e 573
b0c22438 574This variable was added in Perl 5.004.
a0d0e21e 575
b0c22438 576=item $OSNAME
a0d0e21e 577
b0c22438 578=item $^O
579X<$^O> X<$OSNAME>
a0d0e21e 580
b0c22438 581The name of the operating system under which this copy of Perl was
241a59d9 582built, as determined during the configuration process. For examples
b0c22438 583see L<perlport/PLATFORMS>.
a0d0e21e 584
241a59d9 585The value is identical to C<$Config{'osname'}>. See also L<Config>
b0c22438 586and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 587
b0c22438 588In Windows platforms, C<$^O> is not very helpful: since it is always
589C<MSWin32>, it doesn't tell the difference between
241a59d9 59095/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
b0c22438 591Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
592between the variants.
a0d0e21e 593
b0c22438 594This variable was added in Perl 5.003.
a0d0e21e 595
1fa81471
DR
596=item %SIG
597X<%SIG>
a0d0e21e 598
241a59d9 599The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 600
9548c15c
FC
601 sub handler { # 1st argument is signal name
602 my($sig) = @_;
603 print "Caught a SIG$sig--shutting down\n";
604 close(LOG);
605 exit(0);
606 }
a0d0e21e 607
9548c15c
FC
608 $SIG{'INT'} = \&handler;
609 $SIG{'QUIT'} = \&handler;
610 ...
611 $SIG{'INT'} = 'DEFAULT'; # restore default action
612 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 613
1fa81471 614Using a value of C<'IGNORE'> usually has the effect of ignoring the
241a59d9 615signal, except for the C<CHLD> signal. See L<perlipc> for more about
1fa81471 616this special case.
a0d0e21e 617
1fa81471 618Here are some other examples:
a0d0e21e 619
9548c15c
FC
620 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
621 # recommended)
622 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
623 # Plumber
624 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
625 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
626 # return??
a0d0e21e 627
1fa81471
DR
628Be sure not to use a bareword as the name of a signal handler,
629lest you inadvertently call it.
a0d0e21e 630
1fa81471 631If your system has the C<sigaction()> function then signal handlers
241a59d9 632are installed using it. This means you get reliable signal handling.
a0d0e21e 633
60cf4914 634The default delivery policy of signals changed in Perl v5.8.0 from
1fa81471 635immediate (also known as "unsafe") to deferred, also known as "safe
241a59d9 636signals". See L<perlipc> for more information.
a0d0e21e 637
241a59d9 638Certain internal hooks can be also set using the C<%SIG> hash. The
1fa81471 639routine indicated by C<$SIG{__WARN__}> is called when a warning
241a59d9
FC
640message is about to be printed. The warning message is passed as the
641first argument. The presence of a C<__WARN__> hook causes the
642ordinary printing of warnings to C<STDERR> to be suppressed. You can
1fa81471
DR
643use this to save warnings in a variable, or turn warnings into fatal
644errors, like this:
a0d0e21e 645
9548c15c
FC
646 local $SIG{__WARN__} = sub { die $_[0] };
647 eval $proggie;
a8f8344d 648
b0c22438 649As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
650disable warnings using the empty subroutine:
f86702cc 651
9548c15c 652 local $SIG{__WARN__} = sub {};
55602bd2 653
b0c22438 654The routine indicated by C<$SIG{__DIE__}> is called when a fatal
241a59d9
FC
655exception is about to be thrown. The error message is passed as the
656first argument. When a C<__DIE__> hook routine returns, the exception
b0c22438 657processing continues as it would have in the absence of the hook,
c94b42ea
DM
658unless the hook routine itself exits via a C<goto &sub>, a loop exit,
659or a C<die()>. The C<__DIE__> handler is explicitly disabled during
660the call, so that you can die from a C<__DIE__> handler. Similarly
661for C<__WARN__>.
e5218da5 662
4a29ab5e
A
663The C<$SIG{__DIE__}> hook is called even inside an C<eval()>. It was
664never intended to happen this way, but an implementation glitch made
665this possible. This used to be deprecated, as it allowed strange action
666at a distance like rewriting a pending exception in C<$@>. Plans to
667rectify this have been scrapped, as users found that rewriting a
668pending exception is actually a useful feature, and not a bug.
b0c22438 669
670C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
241a59d9 671may be called to report (probable) errors found by the parser. In such
b0c22438 672a case the parser may be in inconsistent state, so any attempt to
673evaluate Perl code from such a handler will probably result in a
241a59d9 674segfault. This means that warnings or errors that result from parsing
b0c22438 675Perl should be used with extreme caution, like this:
e5218da5 676
9548c15c
FC
677 require Carp if defined $^S;
678 Carp::confess("Something wrong") if defined &Carp::confess;
679 die "Something wrong, but could not load Carp to give "
680 . "backtrace...\n\t"
681 . "To see backtrace try starting Perl with -MCarp switch";
e5218da5 682
b0c22438 683Here the first line will load C<Carp> I<unless> it is the parser who
241a59d9
FC
684called the handler. The second line will print backtrace and die if
685C<Carp> was available. The third line will be executed only if C<Carp> was
b0c22438 686not available.
0a378802 687
0b9346e6 688Having to even think about the C<$^S> variable in your exception
241a59d9
FC
689handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
690invites grievous and difficult to track down errors. Avoid it
0b9346e6 691and use an C<END{}> or CORE::GLOBAL::die override instead.
692
b0c22438 693See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
694L<warnings> for additional information.
0a378802 695
b0c22438 696=item $BASETIME
6ab308ee 697
b0c22438 698=item $^T
699X<$^T> X<$BASETIME>
6ab308ee 700
b0c22438 701The time at which the program began running, in seconds since the
241a59d9 702epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
b0c22438 703and B<-C> filetests are based on this value.
a0d0e21e 704
b0c22438 705=item $PERL_VERSION
a0d0e21e 706
b0c22438 707=item $^V
708X<$^V> X<$PERL_VERSION>
a0d0e21e 709
b0c22438 710The revision, version, and subversion of the Perl interpreter,
eb82332c 711represented as a L<version> object.
748a9306 712
60cf4914
BF
713This variable first appeared in perl v5.6.0; earlier versions of perl
714will see an undefined value. Before perl v5.10.0 C<$^V> was represented
f20d3573 715as a v-string rather than a L<version> object.
55602bd2 716
b0c22438 717C<$^V> can be used to determine whether the Perl interpreter executing
241a59d9 718a script is in the right range of versions. For example:
a0d0e21e 719
9548c15c 720 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 721
f20d3573
DG
722While version objects overload stringification, to portably convert
723C<$^V> into its string representation, use C<sprintf()>'s C<"%vd">
724conversion, which works for both v-strings or version objects:
a0d0e21e 725
9548c15c 726 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 727
b0c22438 728See the documentation of C<use VERSION> and C<require VERSION>
729for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 730
6a29646e 731See also C<L</$]>> for a decimal representation of the Perl version.
a0d0e21e 732
f20d3573
DG
733The main advantage of C<$^V> over C<$]> is that, for Perl v5.10.0 or
734later, it overloads operators, allowing easy comparison against other
735version representations (e.g. decimal, literal v-string, "v1.2.3", or
736objects). The disadvantage is that prior to v5.10.0, it was only a
6a29646e
KW
737literal v-string, which can't be easily printed or compared, whereas
738the behavior of C<$]> is unchanged on all versions of Perl.
a0d0e21e 739
eb82332c 740Mnemonic: use ^V for a version object.
a0d0e21e 741
b0c22438 742=item ${^WIN32_SLOPPY_STAT}
5b442a2a 743X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
a0d0e21e 744
b0c22438 745If this variable is set to a true value, then C<stat()> on Windows will
241a59d9 746not try to open the file. This means that the link count cannot be
b0c22438 747determined and file attributes may be out of date if additional
241a59d9 748hardlinks to the file exist. On the other hand, not opening the file
b0c22438 749is considerably faster, especially for files on network drives.
a0d0e21e 750
b0c22438 751This variable could be set in the F<sitecustomize.pl> file to
752configure the local Perl installation to use "sloppy" C<stat()> by
241a59d9 753default. See the documentation for B<-f> in
b0c22438 754L<perlrun|perlrun/"Command Switches"> for more information about site
755customization.
a0d0e21e 756
60cf4914 757This variable was added in Perl v5.10.0.
a0d0e21e 758
b0c22438 759=item $EXECUTABLE_NAME
a0d0e21e 760
b0c22438 761=item $^X
762X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 763
b0c22438 764The name used to execute the current copy of Perl, from C's
765C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 766
b0c22438 767Depending on the host operating system, the value of C<$^X> may be
768a relative or absolute pathname of the perl program file, or may
769be the string used to invoke perl but not the pathname of the
241a59d9 770perl program file. Also, most operating systems permit invoking
b0c22438 771programs that are not in the PATH environment variable, so there
241a59d9 772is no guarantee that the value of C<$^X> is in PATH. For VMS, the
b0c22438 773value may or may not include a version number.
a0d0e21e 774
b0c22438 775You usually can use the value of C<$^X> to re-invoke an independent
776copy of the same perl that is currently running, e.g.,
a0d0e21e 777
9548c15c 778 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 779
b0c22438 780But recall that not all operating systems support forking or
781capturing of the output of commands, so this complex statement
782may not be portable.
a0d0e21e 783
b0c22438 784It is not safe to use the value of C<$^X> as a path name of a file,
785as some operating systems that have a mandatory suffix on
786executable files do not require use of the suffix when invoking
241a59d9 787a command. To convert the value of C<$^X> to a path name, use the
b0c22438 788following statements:
8cc95fdb 789
9548c15c
FC
790 # Build up a set of file names (not command names).
791 use Config;
792 my $this_perl = $^X;
793 if ($^O ne 'VMS') {
794 $this_perl .= $Config{_exe}
795 unless $this_perl =~ m/$Config{_exe}$/i;
796 }
8cc95fdb 797
b0c22438 798Because many operating systems permit anyone with read access to
799the Perl program file to make a copy of it, patch the copy, and
800then execute the copy, the security-conscious Perl programmer
801should take care to invoke the installed copy of perl, not the
241a59d9 802copy referenced by C<$^X>. The following statements accomplish
b0c22438 803this goal, and produce a pathname that can be invoked as a
804command or referenced as a file.
a043a685 805
9548c15c
FC
806 use Config;
807 my $secure_perl_path = $Config{perlpath};
808 if ($^O ne 'VMS') {
809 $secure_perl_path .= $Config{_exe}
810 unless $secure_perl_path =~ m/$Config{_exe}$/i;
811 }
a0d0e21e 812
b0c22438 813=back
a0d0e21e 814
b0c22438 815=head2 Variables related to regular expressions
816
817Most of the special variables related to regular expressions are side
241a59d9
FC
818effects. Perl sets these variables when it has a successful match, so
819you should check the match result before using them. For instance:
b0c22438 820
9548c15c
FC
821 if( /P(A)TT(ER)N/ ) {
822 print "I found $1 and $2\n";
823 }
b0c22438 824
0b9346e6 825These variables are read-only and dynamically-scoped, unless we note
b0c22438 826otherwise.
827
0b9346e6 828The dynamic nature of the regular expression variables means that
829their value is limited to the block that they are in, as demonstrated
830by this bit of code:
b0c22438 831
9548c15c
FC
832 my $outer = 'Wallace and Grommit';
833 my $inner = 'Mutt and Jeff';
0b9346e6 834
9548c15c 835 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 836
9548c15c 837 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 838
9548c15c
FC
839 {
840 OUTER:
841 show_n() if $outer =~ m/$pattern/;
0b9346e6 842
9548c15c
FC
843 INNER: {
844 show_n() if $inner =~ m/$pattern/;
845 }
0b9346e6 846
9548c15c
FC
847 show_n();
848 }
b0c22438 849
0b9346e6 850The output shows that while in the C<OUTER> block, the values of C<$1>
241a59d9 851and C<$2> are from the match against C<$outer>. Inside the C<INNER>
0b9346e6 852block, the values of C<$1> and C<$2> are from the match against
853C<$inner>, but only until the end of the block (i.e. the dynamic
241a59d9 854scope). After the C<INNER> block completes, the values of C<$1> and
0b9346e6 855C<$2> return to the values for the match against C<$outer> even though
b0c22438 856we have not made another match:
857
9548c15c
FC
858 $1 is Wallace; $2 is Grommit
859 $1 is Mutt; $2 is Jeff
860 $1 is Wallace; $2 is Grommit
a0d0e21e 861
40445027 862=head3 Performance issues
0b9346e6 863
40445027
DM
864Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
865or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
866all subsequent successful pattern matches to make a copy of the matched
867string, in case the code might subsequently access one of those variables.
868This imposed a considerable performance penalty across the whole program,
869so generally the use of these variables has been discouraged.
0b9346e6 870
40445027
DM
871In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
872supply the indices of successful matches. So you could for example do
873this:
874
875 $str =~ /pattern/;
876
877 print $`, $&, $'; # bad: perfomance hit
878
879 print # good: no perfomance hit
880 substr($str, 0, $-[0]),
881 substr($str, $-[0], $+[0]-$-[0]),
882 substr($str, $+[0]);
883
884In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
885C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
886you to suffer the penalties only on patterns marked with C</p>.
887
888In Perl 5.18.0 onwards, perl started noting the presence of each of the
889three variables separately, and only copied that part of the string
890required; so in
891
892 $`; $&; "abcdefgh" =~ /d/
893
894perl would only copy the "abcd" part of the string. That could make a big
895difference in something like
896
897 $str = 'x' x 1_000_000;
898 $&; # whoops
899 $str =~ /x/g # one char copied a million times, not a million chars
900
901In Perl 5.20.0 a new copy-on-write system was enabled by default, which
902finally fixes all performance issues with these three variables, and makes
903them safe to use anywhere.
904
905The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
906find uses of these problematic match variables in your code.
13b0f67d 907
b0c22438 908=over 8
a0d0e21e 909
b0c22438 910=item $<I<digits>> ($1, $2, ...)
27deb0cf 911X<$1> X<$2> X<$3> X<$I<digits>>
8cc95fdb 912
b0c22438 913Contains the subpattern from the corresponding set of capturing
914parentheses from the last successful pattern match, not counting patterns
915matched in nested blocks that have been exited already.
8cc95fdb 916
27deb0cf
YO
917Note there is a distinction between a capture buffer which matches
918the empty string a capture buffer which is optional. Eg, C<(x?)> and
919C<(x)?> The latter may be undef, the former not.
920
b0c22438 921These variables are read-only and dynamically-scoped.
a043a685 922
b0c22438 923Mnemonic: like \digits.
a0d0e21e 924
27deb0cf
YO
925=item @{^CAPTURE}
926X<@{^CAPTURE}> X<@^CAPTURE>
927
95013431
YO
928An array which exposes the contents of the capture buffers, if any, of
929the last successful pattern match, not counting patterns matched
27deb0cf
YO
930in nested blocks that have been exited already.
931
932Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1 index
933is equivalent to $2, etc.
934
935 if ("foal"=~/(.)(.)(.)(.)/) {
936 print join "-", @{^CAPTURE};
937 }
938
939should output "f-o-a-l".
940
941See also L</$I<digits>>, L</%{^CAPTURE}> and L</%{^CAPTURE_ALL}>.
942
95013431
YO
943Note that unlike most other regex magic variables there is no single
944letter equivalent to C<@{^CAPTURE}>.
945
27deb0cf
YO
946This variable was added in 5.25.7
947
b0c22438 948=item $MATCH
a0d0e21e 949
b0c22438 950=item $&
951X<$&> X<$MATCH>
a0d0e21e 952
b0c22438 953The string matched by the last successful pattern match (not counting
954any matches hidden within a BLOCK or C<eval()> enclosed by the current
955BLOCK).
a0d0e21e 956
40445027
DM
957See L</Performance issues> above for the serious performance implications
958of using this variable (even once) in your code.
80bca1b4 959
b0c22438 960This variable is read-only and dynamically-scoped.
f9cbb277 961
b0c22438 962Mnemonic: like C<&> in some editors.
0b9346e6 963
b0c22438 964=item ${^MATCH}
965X<${^MATCH}>
a0d0e21e 966
b0c22438 967This is similar to C<$&> (C<$MATCH>) except that it does not incur the
13b0f67d 968performance penalty associated with that variable.
40445027
DM
969
970See L</Performance issues> above.
971
13b0f67d 972In Perl v5.18 and earlier, it is only guaranteed
b0c22438 973to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
974the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
975C<${^MATCH}> does the same thing as C<$MATCH>.
80bca1b4 976
60cf4914 977This variable was added in Perl v5.10.0.
4bc88a62 978
b0c22438 979This variable is read-only and dynamically-scoped.
e2975953 980
b0c22438 981=item $PREMATCH
52c447a8 982
b0c22438 983=item $`
5b442a2a 984X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 985
b0c22438 986The string preceding whatever was matched by the last successful
987pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 988enclosed by the current BLOCK.
a0d0e21e 989
40445027
DM
990See L</Performance issues> above for the serious performance implications
991of using this variable (even once) in your code.
a0d0e21e 992
b0c22438 993This variable is read-only and dynamically-scoped.
a0d0e21e 994
b0c22438 995Mnemonic: C<`> often precedes a quoted string.
f83ed198 996
b0c22438 997=item ${^PREMATCH}
5b442a2a 998X<$`> X<${^PREMATCH}>
a0d0e21e 999
b0c22438 1000This is similar to C<$`> ($PREMATCH) except that it does not incur the
13b0f67d 1001performance penalty associated with that variable.
40445027
DM
1002
1003See L</Performance issues> above.
1004
13b0f67d 1005In Perl v5.18 and earlier, it is only guaranteed
b0c22438 1006to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
1007the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1008C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
a0d0e21e 1009
4a70680a 1010This variable was added in Perl v5.10.0.
a0d0e21e 1011
b0c22438 1012This variable is read-only and dynamically-scoped.
a0d0e21e 1013
b0c22438 1014=item $POSTMATCH
16070b82 1015
b0c22438 1016=item $'
5b442a2a 1017X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 1018
b0c22438 1019The string following whatever was matched by the last successful
1020pattern match (not counting any matches hidden within a BLOCK or C<eval()>
241a59d9 1021enclosed by the current BLOCK). Example:
305aace0 1022
9548c15c
FC
1023 local $_ = 'abcdefghi';
1024 /def/;
1025 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1026
40445027
DM
1027See L</Performance issues> above for the serious performance implications
1028of using this variable (even once) in your code.
a0d0e21e 1029
b0c22438 1030This variable is read-only and dynamically-scoped.
1031
1032Mnemonic: C<'> often follows a quoted string.
1033
1034=item ${^POSTMATCH}
5b442a2a 1035X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 1036
1037This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
13b0f67d 1038performance penalty associated with that variable.
40445027
DM
1039
1040See L</Performance issues> above.
1041
13b0f67d 1042In Perl v5.18 and earlier, it is only guaranteed
b0c22438 1043to return a defined value when the pattern was compiled or executed with
13b0f67d
DM
1044the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1045C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
b0c22438 1046
60cf4914 1047This variable was added in Perl v5.10.0.
b0c22438 1048
1049This variable is read-only and dynamically-scoped.
1050
1051=item $LAST_PAREN_MATCH
1052
1053=item $+
1054X<$+> X<$LAST_PAREN_MATCH>
1055
1056The text matched by the last bracket of the last successful search pattern.
1057This is useful if you don't know which one of a set of alternative patterns
241a59d9 1058matched. For example:
b0c22438 1059
9548c15c 1060 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1061
1062This variable is read-only and dynamically-scoped.
1063
1064Mnemonic: be positive and forward looking.
1065
1066=item $LAST_SUBMATCH_RESULT
1067
1068=item $^N
5b442a2a 1069X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1070
1071The text matched by the used group most-recently closed (i.e. the group
1072with the rightmost closing parenthesis) of the last successful search
1073pattern.
1074
1075This is primarily used inside C<(?{...})> blocks for examining text
241a59d9 1076recently matched. For example, to effectively capture text to a variable
b0c22438 1077(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1078
9548c15c 1079 (?:(...)(?{ $var = $^N }))
b0c22438 1080
1081By setting and then using C<$var> in this way relieves you from having to
1082worry about exactly which numbered set of parentheses they are.
1083
60cf4914 1084This variable was added in Perl v5.8.0.
b0c22438 1085
1086Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1087
1088=item @LAST_MATCH_END
1089
1090=item @+
1091X<@+> X<@LAST_MATCH_END>
1092
1093This array holds the offsets of the ends of the last successful
241a59d9
FC
1094submatches in the currently active dynamic scope. C<$+[0]> is
1095the offset into the string of the end of the entire match. This
b0c22438 1096is the same value as what the C<pos> function returns when called
241a59d9 1097on the variable that was matched against. The I<n>th element
b0c22438 1098of this array holds the offset of the I<n>th submatch, so
1099C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
241a59d9
FC
1100past where C<$2> ends, and so on. You can use C<$#+> to determine
1101how many subgroups were in the last successful match. See the
b0c22438 1102examples given for the C<@-> variable.
1103
60cf4914 1104This variable was added in Perl v5.6.0.
b0c22438 1105
27deb0cf
YO
1106=item %{^CAPTURE}
1107
b0c22438 1108=item %LAST_PAREN_MATCH
1109
1110=item %+
27deb0cf 1111X<%+> X<%LAST_PAREN_MATCH> X<%{^CAPTURE}>
b0c22438 1112
1113Similar to C<@+>, the C<%+> hash allows access to the named capture
1114buffers, should they exist, in the last successful match in the
1115currently active dynamic scope.
1116
1117For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1118
9548c15c 1119 'foo' =~ /(?<foo>foo)/;
b0c22438 1120
1121The keys of the C<%+> hash list only the names of buffers that have
1122captured (and that are thus associated to defined values).
1123
33727e0f
LM
1124If multiple distinct capture groups have the same name, then
1125C<$+{NAME}> will refer to the leftmost defined group in the match.
1126
b0c22438 1127The underlying behaviour of C<%+> is provided by the
1128L<Tie::Hash::NamedCapture> module.
1129
1130B<Note:> C<%-> and C<%+> are tied views into a common internal hash
241a59d9 1131associated with the last successful regular expression. Therefore mixing
b0c22438 1132iterative access to them via C<each> may have unpredictable results.
1133Likewise, if the last successful match changes, then the results may be
1134surprising.
1135
27deb0cf
YO
1136This variable was added in Perl v5.10.0. The C<%{^CAPTURE}> alias was
1137added in 5.25.7.
a0d0e21e 1138
b0c22438 1139This variable is read-only and dynamically-scoped.
1140
1141=item @LAST_MATCH_START
1142
1143=item @-
1144X<@-> X<@LAST_MATCH_START>
1145
1146C<$-[0]> is the offset of the start of the last successful match.
1147C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1148I<n>-th subpattern, or undef if the subpattern did not match.
1149
1150Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
241a59d9 1151$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
b0c22438 1152$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
241a59d9
FC
1153C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1154last matched subgroup in the last successful match. Contrast with
1155C<$#+>, the number of subgroups in the regular expression. Compare
b0c22438 1156with C<@+>.
1157
1158This array holds the offsets of the beginnings of the last
1159successful submatches in the currently active dynamic scope.
1160C<$-[0]> is the offset into the string of the beginning of the
241a59d9 1161entire match. The I<n>th element of this array holds the offset
b0c22438 1162of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1163begins, C<$-[2]> the offset where C<$2> begins, and so on.
1164
1165After a match against some variable C<$var>:
1166
1167=over 5
1168
1169=item C<$`> is the same as C<substr($var, 0, $-[0])>
1170
1171=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1172
1173=item C<$'> is the same as C<substr($var, $+[0])>
1174
1175=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1176
1177=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1178
1179=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1180
1181=back
1182
60cf4914 1183This variable was added in Perl v5.6.0.
b0c22438 1184
27deb0cf
YO
1185=item %{^CAPTURE_ALL}
1186X<%{^CAPTURE_ALL}>
1187
b0c22438 1188=item %-
2e67aae7 1189X<%->
b0c22438 1190
1191Similar to C<%+>, this variable allows access to the named capture groups
241a59d9 1192in the last successful match in the currently active dynamic scope. To
b0c22438 1193each capture group name found in the regular expression, it associates a
1194reference to an array containing the list of values captured by all
1195buffers with that name (should there be several of them), in the order
1196where they appear.
1197
1198Here's an example:
1199
1200 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1201 foreach my $bufname (sort keys %-) {
1202 my $ary = $-{$bufname};
1203 foreach my $idx (0..$#$ary) {
1204 print "\$-{$bufname}[$idx] : ",
9548c15c
FC
1205 (defined($ary->[$idx])
1206 ? "'$ary->[$idx]'"
1207 : "undef"),
b0c22438 1208 "\n";
1209 }
1210 }
1211 }
1212
1213would print out:
1214
9548c15c
FC
1215 $-{A}[0] : '1'
1216 $-{A}[1] : '3'
1217 $-{B}[0] : '2'
1218 $-{B}[1] : '4'
b0c22438 1219
1220The keys of the C<%-> hash correspond to all buffer names found in
1221the regular expression.
1222
1223The behaviour of C<%-> is implemented via the
1224L<Tie::Hash::NamedCapture> module.
1225
1226B<Note:> C<%-> and C<%+> are tied views into a common internal hash
241a59d9 1227associated with the last successful regular expression. Therefore mixing
b0c22438 1228iterative access to them via C<each> may have unpredictable results.
1229Likewise, if the last successful match changes, then the results may be
1230surprising.
1231
27deb0cf
YO
1232This variable was added in Perl v5.10.0. The C<%{^CAPTURE_ALL}> alias was
1233added in 5.25.7.
b0c22438 1234
1235This variable is read-only and dynamically-scoped.
1236
1237=item $LAST_REGEXP_CODE_RESULT
1238
1239=item $^R
1240X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1241
1242The result of evaluation of the last successful C<(?{ code })>
241a59d9 1243regular expression assertion (see L<perlre>). May be written to.
b0c22438 1244
1245This variable was added in Perl 5.005.
a0d0e21e 1246
a3621e74 1247=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1248X<${^RE_DEBUG_FLAGS}>
a3621e74 1249
241a59d9
FC
1250The current value of the regex debugging flags. Set to 0 for no debug output
1251even when the C<re 'debug'> module is loaded. See L<re> for details.
b0c22438 1252
60cf4914 1253This variable was added in Perl v5.10.0.
a3621e74 1254
0111c4fd 1255=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1256X<${^RE_TRIE_MAXBUF}>
a3621e74
YO
1257
1258Controls how certain regex optimisations are applied and how much memory they
241a59d9
FC
1259utilize. This value by default is 65536 which corresponds to a 512kB
1260temporary cache. Set this to a higher value to trade
1261memory for speed when matching large alternations. Set
1262it to a lower value if you want the optimisations to
a3621e74
YO
1263be as conservative of memory as possible but still occur, and set it to a
1264negative value to prevent the optimisation and conserve the most memory.
1265Under normal situations this variable should be of no interest to you.
1266
60cf4914 1267This variable was added in Perl v5.10.0.
a0d0e21e 1268
b0c22438 1269=back
a0d0e21e 1270
b0c22438 1271=head2 Variables related to filehandles
a0d0e21e 1272
b0c22438 1273Variables that depend on the currently selected filehandle may be set
1274by calling an appropriate object method on the C<IO::Handle> object,
1275although this is less efficient than using the regular built-in
241a59d9 1276variables. (Summary lines below for this contain the word HANDLE.)
b0c22438 1277First you must say
6e2995f4 1278
9548c15c 1279 use IO::Handle;
0462a1ab 1280
b0c22438 1281after which you may use either
0462a1ab 1282
9548c15c 1283 method HANDLE EXPR
0462a1ab 1284
b0c22438 1285or more safely,
0462a1ab 1286
9548c15c 1287 HANDLE->method(EXPR)
0462a1ab 1288
241a59d9 1289Each method returns the old value of the C<IO::Handle> attribute. The
b0c22438 1290methods each take an optional EXPR, which, if supplied, specifies the
241a59d9 1291new value for the C<IO::Handle> attribute in question. If not
b0c22438 1292supplied, most methods do nothing to the current value--except for
1293C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1294
b0c22438 1295Because loading in the C<IO::Handle> class is an expensive operation,
1296you should learn how to use the regular built-in variables.
1297
241a59d9 1298A few of these variables are considered "read-only". This means that
b0c22438 1299if you try to assign to this variable, either directly or indirectly
1300through a reference, you'll raise a run-time exception.
1301
1302You should be very careful when modifying the default values of most
241a59d9 1303special variables described in this document. In most cases you want
b0c22438 1304to localize these variables before changing them, since if you don't,
1305the change may affect other modules which rely on the default values
241a59d9 1306of the special variables that you have changed. This is one of the
b0c22438 1307correct ways to read the whole file at once:
1308
9548c15c
FC
1309 open my $fh, "<", "foo" or die $!;
1310 local $/; # enable localized slurp mode
1311 my $content = <$fh>;
1312 close $fh;
b0c22438 1313
1314But the following code is quite bad:
1315
9548c15c
FC
1316 open my $fh, "<", "foo" or die $!;
1317 undef $/; # enable slurp mode
1318 my $content = <$fh>;
1319 close $fh;
b0c22438 1320
1321since some other module, may want to read data from some file in the
1322default "line mode", so if the code we have just presented has been
1323executed, the global value of C<$/> is now changed for any other code
1324running inside the same Perl interpreter.
1325
1326Usually when a variable is localized you want to make sure that this
241a59d9
FC
1327change affects the shortest scope possible. So unless you are already
1328inside some short C<{}> block, you should create one yourself. For
b0c22438 1329example:
1330
9548c15c
FC
1331 my $content = '';
1332 open my $fh, "<", "foo" or die $!;
1333 {
1334 local $/;
1335 $content = <$fh>;
1336 }
1337 close $fh;
0462a1ab 1338
b0c22438 1339Here is an example of how your own code can go broken:
0462a1ab 1340
9548c15c
FC
1341 for ( 1..3 ){
1342 $\ = "\r\n";
1343 nasty_break();
1344 print "$_";
1345 }
0b9346e6 1346
9548c15c 1347 sub nasty_break {
0b9346e6 1348 $\ = "\f";
1349 # do something with $_
9548c15c 1350 }
0462a1ab 1351
0b9346e6 1352You probably expect this code to print the equivalent of
0462a1ab 1353
0b9346e6 1354 "1\r\n2\r\n3\r\n"
0462a1ab 1355
b0c22438 1356but instead you get:
0462a1ab 1357
0b9346e6 1358 "1\f2\f3\f"
0462a1ab 1359
0b9346e6 1360Why? Because C<nasty_break()> modifies C<$\> without localizing it
241a59d9
FC
1361first. The value you set in C<nasty_break()> is still there when you
1362return. The fix is to add C<local()> so the value doesn't leak out of
0b9346e6 1363C<nasty_break()>:
6e2995f4 1364
9548c15c 1365 local $\ = "\f";
a0d0e21e 1366
b0c22438 1367It's easy to notice the problem in such a short example, but in more
1368complicated code you are looking for trouble if you don't localize
1369changes to the special variables.
a0d0e21e 1370
b0c22438 1371=over 8
a0d0e21e 1372
b0c22438 1373=item $ARGV
1374X<$ARGV>
fb73857a 1375
ca1b95ae 1376Contains the name of the current file when reading from C<< <> >>.
b0c22438 1377
1378=item @ARGV
1379X<@ARGV>
1380
ca1b95ae 1381The array C<@ARGV> contains the command-line arguments intended for
241a59d9 1382the script. C<$#ARGV> is generally the number of arguments minus
b0c22438 1383one, because C<$ARGV[0]> is the first argument, I<not> the program's
241a59d9 1384command name itself. See L</$0> for the command name.
b0c22438 1385
84dabc03 1386=item ARGV
1387X<ARGV>
1388
1389The special filehandle that iterates over command-line filenames in
241a59d9
FC
1390C<@ARGV>. Usually written as the null filehandle in the angle operator
1391C<< <> >>. Note that currently C<ARGV> only has its magical effect
84dabc03 1392within the C<< <> >> operator; elsewhere it is just a plain filehandle
241a59d9 1393corresponding to the last file opened by C<< <> >>. In particular,
84dabc03 1394passing C<\*ARGV> as a parameter to a function that expects a filehandle
1395may not cause your function to automatically read the contents of all the
1396files in C<@ARGV>.
1397
b0c22438 1398=item ARGVOUT
1399X<ARGVOUT>
1400
1401The special filehandle that points to the currently open output file
241a59d9
FC
1402when doing edit-in-place processing with B<-i>. Useful when you have
1403to do a lot of inserting and don't want to keep modifying C<$_>. See
b0c22438 1404L<perlrun> for the B<-i> switch.
1405
96948506 1406=item IO::Handle->output_field_separator( EXPR )
84dabc03 1407
1408=item $OUTPUT_FIELD_SEPARATOR
1409
1410=item $OFS
1411
1412=item $,
1413X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1414
241a59d9
FC
1415The output field separator for the print operator. If defined, this
1416value is printed between each of print's arguments. Default is C<undef>.
84dabc03 1417
96948506 1418You cannot call C<output_field_separator()> on a handle, only as a
008f9687 1419static method. See L<IO::Handle|IO::Handle>.
96948506 1420
84dabc03 1421Mnemonic: what is printed when there is a "," in your print statement.
1422
5b442a2a 1423=item HANDLE->input_line_number( EXPR )
b0c22438 1424
1425=item $INPUT_LINE_NUMBER
1426
1427=item $NR
1428
1429=item $.
1430X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1431
1432Current line number for the last filehandle accessed.
1433
1434Each filehandle in Perl counts the number of lines that have been read
241a59d9 1435from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1436constitutes a line may not match yours.) When a line is read from a
1437filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1438C<seek()> is called on it, C<$.> becomes an alias to the line counter
1439for that filehandle.
1440
1441You can adjust the counter by assigning to C<$.>, but this will not
241a59d9
FC
1442actually move the seek pointer. I<Localizing C<$.> will not localize
1443the filehandle's line count>. Instead, it will localize perl's notion
b0c22438 1444of which filehandle C<$.> is currently aliased to.
1445
1446C<$.> is reset when the filehandle is closed, but B<not> when an open
241a59d9
FC
1447filehandle is reopened without an intervening C<close()>. For more
1448details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
b0c22438 1449an explicit close, line numbers increase across C<ARGV> files (but see
1450examples in L<perlfunc/eof>).
1451
1452You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1453line counter for a given filehandle without having to worry about
1454which handle you last accessed.
1455
1456Mnemonic: many programs use "." to mean the current line number.
1457
96948506 1458=item IO::Handle->input_record_separator( EXPR )
b0c22438 1459
1460=item $INPUT_RECORD_SEPARATOR
1461
1462=item $RS
1463
1464=item $/
1465X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1466
241a59d9
FC
1467The input record separator, newline by default. This influences Perl's
1468idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1469treating empty lines as a terminator if set to the null string (an
241a59d9 1470empty line cannot contain any spaces or tabs). You may set it to a
84dabc03 1471multi-character string to match a multi-character terminator, or to
241a59d9 1472C<undef> to read through the end of file. Setting it to C<"\n\n">
84dabc03 1473means something slightly different than setting to C<"">, if the file
241a59d9
FC
1474contains consecutive empty lines. Setting to C<""> will treat two or
1475more consecutive empty lines as a single empty line. Setting to
84dabc03 1476C<"\n\n"> will blindly assume that the next input character belongs to
1477the next paragraph, even if it's a newline.
b0c22438 1478
1479 local $/; # enable "slurp" mode
1480 local $_ = <FH>; # whole file now here
1481 s/\n[ \t]+/ /g;
1482
241a59d9 1483Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1484be better for something. :-)
1485
1486Setting C<$/> to a reference to an integer, scalar containing an
1487integer, or scalar that's convertible to an integer will attempt to
1488read records instead of lines, with the maximum record size being the
3d249121 1489referenced integer number of characters. So this:
b0c22438 1490
1491 local $/ = \32768; # or \"32768", or \$var_containing_32768
1492 open my $fh, "<", $myfile or die $!;
1493 local $_ = <$fh>;
fb73857a 1494
f1ee460b 1495will read a record of no more than 32768 characters from $fh. If you're
b0c22438 1496not reading from a record-oriented file (or your OS doesn't have
1497record-oriented files), then you'll likely get a full chunk of data
241a59d9
FC
1498with every read. If a record is larger than the record size you've
1499set, you'll get the record back in pieces. Trying to set the record
b3a2acfa
YO
1500size to zero or less is deprecated and will cause $/ to have the value
1501of "undef", which will cause reading in the (rest of the) whole file.
1502
1503As of 5.19.9 setting C<$/> to any other form of reference will throw a
1504fatal exception. This is in preparation for supporting new ways to set
1505C<$/> in the future.
6e2995f4 1506
78c28381 1507On VMS only, record reads bypass PerlIO layers and any associated
3d249121 1508buffering, so you must not mix record and non-record reads on the
78c28381
CB
1509same filehandle. Record mode mixes with line mode only when the
1510same buffering layer is in use for both modes.
5c055ba3 1511
96948506 1512You cannot call C<input_record_separator()> on a handle, only as a
008f9687 1513static method. See L<IO::Handle|IO::Handle>.
96948506 1514
008f9687 1515See also L<perlport/"Newlines">. Also see L</$.>.
9bf22702 1516
b0c22438 1517Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1518
96948506 1519=item IO::Handle->output_record_separator( EXPR )
84902520 1520
b0c22438 1521=item $OUTPUT_RECORD_SEPARATOR
84902520 1522
b0c22438 1523=item $ORS
84902520 1524
b0c22438 1525=item $\
1526X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
84902520 1527
241a59d9
FC
1528The output record separator for the print operator. If defined, this
1529value is printed after the last of print's arguments. Default is C<undef>.
84902520 1530
96948506 1531You cannot call C<output_record_separator()> on a handle, only as a
008f9687 1532static method. See L<IO::Handle|IO::Handle>.
96948506 1533
b0c22438 1534Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1535Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1536
5b442a2a 1537=item HANDLE->autoflush( EXPR )
1538
1539=item $OUTPUT_AUTOFLUSH
1540
84dabc03 1541=item $|
1542X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1543
84dabc03 1544If set to nonzero, forces a flush right away and after every write or
241a59d9 1545print on the currently selected output channel. Default is 0
84dabc03 1546(regardless of whether the channel is really buffered by the system or
1547not; C<$|> tells you only whether you've asked Perl explicitly to
241a59d9
FC
1548flush after each write). STDOUT will typically be line buffered if
1549output is to the terminal and block buffered otherwise. Setting this
84dabc03 1550variable is useful primarily when you are outputting to a pipe or
1551socket, such as when you are running a Perl program under B<rsh> and
241a59d9
FC
1552want to see the output as it's happening. This has no effect on input
1553buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1554how to select the output channel. See also L<IO::Handle>.
84dabc03 1555
1556Mnemonic: when you want your pipes to be piping hot.
1557
8561ea1d
FC
1558=item ${^LAST_FH}
1559X<${^LAST_FH}>
1560
1561This read-only variable contains a reference to the last-read filehandle.
1562This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1563This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1564use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1565an error or warning message.
1566
1567This variable was added in Perl v5.18.0.
1568
84dabc03 1569=back
84902520 1570
b0c22438 1571=head3 Variables related to formats
83ee9e09 1572
b0c22438 1573The special variables for formats are a subset of those for
241a59d9 1574filehandles. See L<perlform> for more information about Perl's
69b55ccc 1575formats.
83ee9e09 1576
b0c22438 1577=over 8
83ee9e09 1578
84dabc03 1579=item $ACCUMULATOR
1580
1581=item $^A
1582X<$^A> X<$ACCUMULATOR>
1583
1584The current value of the C<write()> accumulator for C<format()> lines.
1585A format contains C<formline()> calls that put their result into
241a59d9
FC
1586C<$^A>. After calling its format, C<write()> prints out the contents
1587of C<$^A> and empties. So you never really see the contents of C<$^A>
1588unless you call C<formline()> yourself and then look at it. See
96090e4f 1589L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
84dabc03 1590
96948506 1591=item IO::Handle->format_formfeed(EXPR)
5b442a2a 1592
1593=item $FORMAT_FORMFEED
1594
84dabc03 1595=item $^L
1596X<$^L> X<$FORMAT_FORMFEED>
1597
241a59d9 1598What formats output as a form feed. The default is C<\f>.
84dabc03 1599
96948506 1600You cannot call C<format_formfeed()> on a handle, only as a static
008f9687 1601method. See L<IO::Handle|IO::Handle>.
96948506 1602
b0c22438 1603=item HANDLE->format_page_number(EXPR)
83ee9e09 1604
b0c22438 1605=item $FORMAT_PAGE_NUMBER
83ee9e09 1606
b0c22438 1607=item $%
1608X<$%> X<$FORMAT_PAGE_NUMBER>
83ee9e09 1609
b0c22438 1610The current page number of the currently selected output channel.
83ee9e09 1611
b0c22438 1612Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1613
b0c22438 1614=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1615
b0c22438 1616=item $FORMAT_LINES_LEFT
66558a10 1617
b0c22438 1618=item $-
1619X<$-> X<$FORMAT_LINES_LEFT>
fb73857a 1620
b0c22438 1621The number of lines left on the page of the currently selected output
1622channel.
fa05a9fd 1623
b0c22438 1624Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1625
96948506 1626=item IO::Handle->format_line_break_characters EXPR
fb73857a 1627
84dabc03 1628=item $FORMAT_LINE_BREAK_CHARACTERS
a0d0e21e 1629
84dabc03 1630=item $:
1631X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e 1632
84dabc03 1633The current set of characters after which a string may be broken to
241a59d9 1634fill continuation fields (starting with C<^>) in a format. The default is
84dabc03 1635S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1636
96948506 1637You cannot call C<format_line_break_characters()> on a handle, only as
008f9687 1638a static method. See L<IO::Handle|IO::Handle>.
96948506 1639
84dabc03 1640Mnemonic: a "colon" in poetry is a part of a line.
1641
1642=item HANDLE->format_lines_per_page(EXPR)
1643
1644=item $FORMAT_LINES_PER_PAGE
1645
1646=item $=
1647X<$=> X<$FORMAT_LINES_PER_PAGE>
1648
1649The current page length (printable lines) of the currently selected
241a59d9 1650output channel. The default is 60.
84dabc03 1651
1652Mnemonic: = has horizontal lines.
7c36658b 1653
b0c22438 1654=item HANDLE->format_top_name(EXPR)
7c36658b 1655
b0c22438 1656=item $FORMAT_TOP_NAME
a05d7ebb 1657
b0c22438 1658=item $^
1659X<$^> X<$FORMAT_TOP_NAME>
fde18df1 1660
b0c22438 1661The name of the current top-of-page format for the currently selected
241a59d9
FC
1662output channel. The default is the name of the filehandle with C<_TOP>
1663appended. For example, the default format top name for the C<STDOUT>
12abbafd 1664filehandle is C<STDOUT_TOP>.
e07ea26a 1665
b0c22438 1666Mnemonic: points to top of page.
e07ea26a 1667
84dabc03 1668=item HANDLE->format_name(EXPR)
16070b82 1669
84dabc03 1670=item $FORMAT_NAME
aa2f2a36 1671
84dabc03 1672=item $~
1673X<$~> X<$FORMAT_NAME>
aa2f2a36 1674
84dabc03 1675The name of the current report format for the currently selected
241a59d9
FC
1676output channel. The default format name is the same as the filehandle
1677name. For example, the default format name for the C<STDOUT>
84dabc03 1678filehandle is just C<STDOUT>.
16070b82 1679
84dabc03 1680Mnemonic: brother to C<$^>.
16070b82 1681
b0c22438 1682=back
a0d0e21e 1683
84dabc03 1684=head2 Error Variables
b0c22438 1685X<error> X<exception>
a0d0e21e 1686
b0c22438 1687The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1688about different types of error conditions that may appear during
241a59d9 1689execution of a Perl program. The variables are shown ordered by
b0c22438 1690the "distance" between the subsystem which reported the error and
241a59d9 1691the Perl process. They correspond to errors detected by the Perl
b0c22438 1692interpreter, C library, operating system, or an external program,
1693respectively.
4438c4b7 1694
b0c22438 1695To illustrate the differences between these variables, consider the
241a59d9 1696following Perl expression, which uses a single-quoted string. After
7fd683ff 1697execution of this statement, perl may have set all four special error
7333b1c4 1698variables:
4438c4b7 1699
9548c15c
FC
1700 eval q{
1701 open my $pipe, "/cdrom/install |" or die $!;
1702 my @res = <$pipe>;
1703 close $pipe or die "bad pipe: $?, $!";
1704 };
a0d0e21e 1705
7333b1c4 1706When perl executes the C<eval()> expression, it translates the
1707C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
241a59d9 1708and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1709the C library's C<errno> if one of these calls fails.
2a8c8378 1710
84dabc03 1711C<$@> is set if the string to be C<eval>-ed did not compile (this may
1712happen if C<open> or C<close> were imported with bad prototypes), or
241a59d9 1713if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1714value of C<$@> is the compile error, or the argument to C<die> (which
241a59d9 1715will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1716
84dabc03 1717Under a few operating systems, C<$^E> may contain a more verbose error
241a59d9 1718indicator, such as in this case, "CDROM tray not closed." Systems that
84dabc03 1719do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1720
2e6ba115 1721Finally, C<$?> may be set to a non-0 value if the external program
241a59d9 1722F</cdrom/install> fails. The upper eight bits reflect specific error
84dabc03 1723conditions encountered by the program (the program's C<exit()> value).
1724The lower eight bits reflect mode of failure, like signal death and
241a59d9 1725core dump information. See L<wait(2)> for details. In contrast to
2e6ba115 1726C<$!> and C<$^E>, which are set only if an error condition is detected,
84dabc03 1727the variable C<$?> is set on each C<wait> or pipe C<close>,
241a59d9 1728overwriting the old value. This is more like C<$@>, which on every
84dabc03 1729C<eval()> is always set on failure and cleared on success.
a0d0e21e 1730
b0c22438 1731For more details, see the individual descriptions at C<$@>, C<$!>,
1732C<$^E>, and C<$?>.
38e4f4ae 1733
0b9346e6 1734=over 8
1735
b0c22438 1736=item ${^CHILD_ERROR_NATIVE}
1737X<$^CHILD_ERROR_NATIVE>
a0d0e21e 1738
b0c22438 1739The native status returned by the last pipe close, backtick (C<``>)
1740command, successful call to C<wait()> or C<waitpid()>, or from the
241a59d9 1741C<system()> operator. On POSIX-like systems this value can be decoded
b0c22438 1742with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1743WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1744
b0c22438 1745Under VMS this reflects the actual VMS exit status; i.e. it is the
1746same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1747
60cf4914 1748This variable was added in Perl v5.10.0.
a0d0e21e 1749
5b442a2a 1750=item $EXTENDED_OS_ERROR
1751
84dabc03 1752=item $^E
1753X<$^E> X<$EXTENDED_OS_ERROR>
1754
241a59d9 1755Error information specific to the current operating system. At the
a804e657 1756moment, this differs from C<L</$!>> under only VMS, OS/2, and Win32 (and
241a59d9 1757for MacPerl). On all other platforms, C<$^E> is always just the same
84dabc03 1758as C<$!>.
1759
1760Under VMS, C<$^E> provides the VMS status value from the last system
241a59d9
FC
1761error. This is more specific information about the last system error
1762than that provided by C<$!>. This is particularly important when C<$!>
84dabc03 1763is set to B<EVMSERR>.
1764
1765Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1766API either via CRT, or directly from perl.
1767
1768Under Win32, C<$^E> always returns the last error information reported
1769by the Win32 call C<GetLastError()> which describes the last error
241a59d9
FC
1770from within the Win32 API. Most Win32-specific code will report errors
1771via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
84dabc03 1772portable Perl code will report errors via C<$!>.
1773
a95b3d6a 1774Caveats mentioned in the description of C<L</$!>> generally apply to
84dabc03 1775C<$^E>, also.
1776
1777This variable was added in Perl 5.003.
1778
1779Mnemonic: Extra error explanation.
0b9346e6 1780
84dabc03 1781=item $EXCEPTIONS_BEING_CAUGHT
1782
1783=item $^S
1784X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1785
1786Current state of the interpreter.
1787
ca1b95ae 1788 $^S State
aa959a20
FC
1789 --------- -------------------------------------
1790 undef Parsing module, eval, or main program
ca1b95ae 1791 true (1) Executing an eval
1792 false (0) Otherwise
84dabc03 1793
1794The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1795handlers.
1796
aa959a20
FC
1797The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1798the C<undef> value does not indicate whether exceptions are being caught,
1799since compilation of the main program does not catch exceptions.
1800
84dabc03 1801This variable was added in Perl 5.004.
1802
1803=item $WARNING
1804
1805=item $^W
1806X<$^W> X<$WARNING>
1807
1808The current value of the warning switch, initially true if B<-w> was
1809used, false otherwise, but directly modifiable.
1810
1811See also L<warnings>.
1812
0b9346e6 1813Mnemonic: related to the B<-w> switch.
84dabc03 1814
1815=item ${^WARNING_BITS}
ca1b95ae 1816X<${^WARNING_BITS}>
84dabc03 1817
1818The current set of warning checks enabled by the C<use warnings> pragma.
44567c86
FC
1819It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1820values are considered internal to the L<warnings> pragma and may change
1821between versions of Perl.
84dabc03 1822
60cf4914 1823This variable was added in Perl v5.6.0.
84dabc03 1824
b0c22438 1825=item $OS_ERROR
5ccee41e 1826
b0c22438 1827=item $ERRNO
5ccee41e 1828
b0c22438 1829=item $!
1830X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1831
a73bef78
JL
1832When referenced, C<$!> retrieves the current value
1833of the C C<errno> integer variable.
1834If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1835When referenced as a string, C<$!> yields the system error string
1836corresponding to C<errno>.
1837
1838Many system or library calls set C<errno> if they fail,
1839to indicate the cause of failure. They usually do B<not>
1840set C<errno> to zero if they succeed. This means C<errno>,
1841hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1842
1843 if (open my $fh, "<", $filename) {
ca1b95ae 1844 # Here $! is meaningless.
1845 ...
7fd683ff 1846 }
ca1b95ae 1847 else {
1848 # ONLY here is $! meaningful.
1849 ...
1850 # Already here $! might be meaningless.
b0c22438 1851 }
1852 # Since here we might have either success or failure,
a73bef78 1853 # $! is meaningless.
a0d0e21e 1854
a73bef78
JL
1855Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1856of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1857It can be used immediately before invoking the C<die()> operator,
1858to set the exit value, or to inspect the system error string
1859corresponding to error I<n>, or to restore C<$!> to a meaningful state.
d54b56d5 1860
b0c22438 1861Mnemonic: What just went bang?
314d39ce 1862
b0c22438 1863=item %OS_ERROR
fb73857a 1864
b0c22438 1865=item %ERRNO
fb73857a 1866
b0c22438 1867=item %!
5b442a2a 1868X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1869
b0c22438 1870Each element of C<%!> has a true value only if C<$!> is set to that
241a59d9 1871value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1872value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1873such file or directory" (or its moral equivalent: not all operating
3b90fd91
RS
1874systems give that exact error, and certainly not all languages). The
1875specific true value is not guaranteed, but in the past has generally
1876been the numeric value of C<$!>. To check if a particular key is
1877meaningful on your system, use C<exists $!{the_key}>; for a list of legal
1878keys, use C<keys %!>. See L<Errno> for more information, and also see
1879L</$!>.
a0d0e21e 1880
b0c22438 1881This variable was added in Perl 5.005.
44f0be63 1882
84dabc03 1883=item $CHILD_ERROR
b687b08b 1884
84dabc03 1885=item $?
1886X<$?> X<$CHILD_ERROR>
a0d0e21e 1887
84dabc03 1888The status returned by the last pipe close, backtick (C<``>) command,
1889successful call to C<wait()> or C<waitpid()>, or from the C<system()>
241a59d9 1890operator. This is just the 16-bit status word returned by the
84dabc03 1891traditional Unix C<wait()> system call (or else is made up to look
241a59d9 1892like it). Thus, the exit value of the subprocess is really (C<<< $? >>
84dabc03 18938 >>>), and C<$? & 127> gives which signal, if any, the process died
1894from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1895
84dabc03 1896Additionally, if the C<h_errno> variable is supported in C, its value
1897is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1898
84dabc03 1899If you have installed a signal handler for C<SIGCHLD>, the
1900value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1901
84dabc03 1902Inside an C<END> subroutine C<$?> contains the value that is going to be
241a59d9
FC
1903given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1904change the exit status of your program. For example:
a0d0e21e 1905
84dabc03 1906 END {
1907 $? = 1 if $? == 255; # die would make it 255
1908 }
a0d0e21e 1909
84dabc03 1910Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1911actual VMS exit status, instead of the default emulation of POSIX
1912status; see L<perlvms/$?> for details.
1913
1914Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1915
b0c22438 1916=item $EVAL_ERROR
f648820c 1917
b0c22438 1918=item $@
1919X<$@> X<$EVAL_ERROR>
a0d0e21e 1920
2e6ba115
LM
1921The Perl error from the last C<eval> operator, i.e. the last exception that
1922was caught. For C<eval BLOCK>, this is either a runtime error message or the
1923string or reference C<die> was called with. The C<eval STRING> form also
1924catches syntax errors and other compile time exceptions.
1925
1926If no error occurs, C<eval> sets C<$@> to the empty string.
a0d0e21e 1927
241a59d9 1928Warning messages are not collected in this variable. You can, however,
b0c22438 1929set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 1930described in L</%SIG>.
748a9306 1931
2e6ba115 1932Mnemonic: Where was the error "at"?
7f315d2e 1933
b0c22438 1934=back
7f315d2e 1935
1fa81471
DR
1936=head2 Variables related to the interpreter state
1937
1938These variables provide information about the current interpreter state.
1939
1940=over 8
1941
1942=item $COMPILING
1943
1944=item $^C
1945X<$^C> X<$COMPILING>
1946
1947The current value of the flag associated with the B<-c> switch.
1948Mainly of use with B<-MO=...> to allow code to alter its behavior
1949when being compiled, such as for example to C<AUTOLOAD> at compile
241a59d9 1950time rather than normal, deferred loading. Setting
1fa81471
DR
1951C<$^C = 1> is similar to calling C<B::minus_c>.
1952
60cf4914 1953This variable was added in Perl v5.6.0.
1fa81471
DR
1954
1955=item $DEBUGGING
1956
1957=item $^D
1958X<$^D> X<$DEBUGGING>
1959
241a59d9 1960The current value of the debugging flags. May be read or set. Like its
a975eeba 1961L<command-line equivalent|perlrun/B<-D>I<letters>>, you can use numeric
8af589bf
KW
1962or symbolic values, e.g. C<$^D = 10> or C<$^D = "st">. See
1963L<perlrun/B<-D>I<number>>. The contents of this variable also affects the
1964debugger operation. See L<perldebguts/Debugger Internals>.
1fa81471
DR
1965
1966Mnemonic: value of B<-D> switch.
1967
1968=item ${^ENCODING}
1969X<${^ENCODING}>
1970
981b911e 1971This variable is no longer supported.
a3ee04ba 1972
981b911e
FC
1973It used to hold the I<object reference> to the C<Encode> object that was
1974used to convert the source code to Unicode.
a3ee04ba 1975
981b911e
FC
1976Its purpose was to allow your non-ASCII Perl
1977scripts not to have to be written in UTF-8; this was
a3ee04ba 1978useful before editors that worked on UTF-8 encoded text were common, but
981b911e
FC
1979that was long ago. It caused problems, such as affecting the operation
1980of other modules that weren't expecting it, causing general mayhem.
a3ee04ba 1981
981b911e
FC
1982If you need something like this functionality, it is recommended that use
1983you a simple source filter, such as L<Filter::Encoding>.
a3ee04ba
KW
1984
1985If you are coming here because code of yours is being adversely affected
1986by someone's use of this variable, you can usually work around it by
1987doing this:
1988
1989 local ${^ENCODING};
1990
1991near the beginning of the functions that are getting broken. This
1992undefines the variable during the scope of execution of the including
1993function.
1fa81471 1994
981b911e 1995This variable was added in Perl 5.8.2 and removed in 5.26.0.
fd503f5c 1996Setting it to anything other than C<undef> was made fatal in Perl 5.28.0.
1fa81471
DR
1997
1998=item ${^GLOBAL_PHASE}
1999X<${^GLOBAL_PHASE}>
2000
2001The current phase of the perl interpreter.
2002
2003Possible values are:
2004
2005=over 8
2006
2007=item CONSTRUCT
2008
241a59d9 2009The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1fa81471 2010value is mostly there for completeness and for use via the
241a59d9 2011underlying C variable C<PL_phase>. It's not really possible for Perl
1fa81471
DR
2012code to be executed unless construction of the interpreter is
2013finished.
2014
2015=item START
2016
241a59d9 2017This is the global compile-time. That includes, basically, every
1fa81471
DR
2018C<BEGIN> block executed directly or indirectly from during the
2019compile-time of the top-level program.
2020
2021This phase is not called "BEGIN" to avoid confusion with
2022C<BEGIN>-blocks, as those are executed during compile-time of any
241a59d9 2023compilation unit, not just the top-level program. A new, localised
1fa81471
DR
2024compile-time entered at run-time, for example by constructs as
2025C<eval "use SomeModule"> are not global interpreter phases, and
2026therefore aren't reflected by C<${^GLOBAL_PHASE}>.
2027
2028=item CHECK
2029
2030Execution of any C<CHECK> blocks.
2031
2032=item INIT
2033
2034Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
2035
2036=item RUN
2037
2038The main run-time, i.e. the execution of C<PL_main_root>.
2039
2040=item END
2041
2042Execution of any C<END> blocks.
2043
2044=item DESTRUCT
2045
2046Global destruction.
2047
2048=back
2049
241a59d9 2050Also note that there's no value for UNITCHECK-blocks. That's because
1fa81471
DR
2051those are run for each compilation unit individually, and therefore is
2052not a global interpreter phase.
2053
2054Not every program has to go through each of the possible phases, but
2055transition from one phase to another can only happen in the order
2056described in the above list.
2057
2058An example of all of the phases Perl code can see:
2059
2060 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
2061
2062 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
2063
2064 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
2065
2066 {
2067 package Print::Phase;
2068
2069 sub new {
2070 my ($class, $time) = @_;
2071 return bless \$time, $class;
2072 }
2073
2074 sub DESTROY {
2075 my $self = shift;
2076 print "$$self: ${^GLOBAL_PHASE}\n";
2077 }
2078 }
2079
2080 print "run-time: ${^GLOBAL_PHASE}\n";
2081
2082 my $runtime = Print::Phase->new(
2083 "lexical variables are garbage collected before END"
2084 );
2085
2086 END { print "end-time: ${^GLOBAL_PHASE}\n" }
2087
2088 our $destruct = Print::Phase->new(
2089 "package variables are garbage collected after END"
2090 );
2091
2092This will print out
2093
2094 compile-time: START
2095 check-time: CHECK
2096 init-time: INIT
2097 run-time: RUN
2098 lexical variables are garbage collected before END: RUN
2099 end-time: END
2100 package variables are garbage collected after END: DESTRUCT
2101
2102This variable was added in Perl 5.14.0.
2103
2104=item $^H
2105X<$^H>
2106
241a59d9
FC
2107WARNING: This variable is strictly for
2108internal use only. Its availability,
1fa81471
DR
2109behavior, and contents are subject to change without notice.
2110
241a59d9 2111This variable contains compile-time hints for the Perl interpreter. At the
1fa81471
DR
2112end of compilation of a BLOCK the value of this variable is restored to the
2113value when the interpreter started to compile the BLOCK.
2114
2115When perl begins to parse any block construct that provides a lexical scope
2116(e.g., eval body, required file, subroutine body, loop body, or conditional
2117block), the existing value of C<$^H> is saved, but its value is left unchanged.
2118When the compilation of the block is completed, it regains the saved value.
2119Between the points where its value is saved and restored, code that
2120executes within BEGIN blocks is free to change the value of C<$^H>.
2121
2122This behavior provides the semantic of lexical scoping, and is used in,
2123for instance, the C<use strict> pragma.
2124
2125The contents should be an integer; different bits of it are used for
241a59d9 2126different pragmatic flags. Here's an example:
1fa81471 2127
9548c15c 2128 sub add_100 { $^H |= 0x100 }
1fa81471 2129
9548c15c
FC
2130 sub foo {
2131 BEGIN { add_100() }
2132 bar->baz($boon);
2133 }
1fa81471 2134
241a59d9 2135Consider what happens during execution of the BEGIN block. At this point
1fa81471 2136the BEGIN block has already been compiled, but the body of C<foo()> is still
241a59d9
FC
2137being compiled. The new value of C<$^H>
2138will therefore be visible only while
1fa81471
DR
2139the body of C<foo()> is being compiled.
2140
2141Substitution of C<BEGIN { add_100() }> block with:
2142
9548c15c 2143 BEGIN { require strict; strict->import('vars') }
1fa81471 2144
241a59d9 2145demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1fa81471
DR
2146version of the same lexical pragma:
2147
9548c15c
FC
2148 BEGIN {
2149 require strict; strict->import('vars') if $condition
2150 }
1fa81471
DR
2151
2152This variable was added in Perl 5.003.
2153
2154=item %^H
2155X<%^H>
2156
241a59d9
FC
2157The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2158it useful for implementation of lexically scoped pragmas. See
112284f4
KW
2159L<perlpragma>. All the entries are stringified when accessed at
2160runtime, so only simple values can be accommodated. This means no
2161pointers to objects, for example.
1fa81471
DR
2162
2163When putting items into C<%^H>, in order to avoid conflicting with other
2164users of the hash there is a convention regarding which keys to use.
2165A module should use only keys that begin with the module's name (the
2166name of its main package) and a "/" character. For example, a module
2167C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2168
60cf4914 2169This variable was added in Perl v5.6.0.
1fa81471
DR
2170
2171=item ${^OPEN}
2172X<${^OPEN}>
2173
241a59d9 2174An internal variable used by PerlIO. A string in two parts, separated
1fa81471
DR
2175by a C<\0> byte, the first part describes the input layers, the second
2176part describes the output layers.
2177
60cf4914 2178This variable was added in Perl v5.8.0.
1fa81471
DR
2179
2180=item $PERLDB
2181
2182=item $^P
2183X<$^P> X<$PERLDB>
2184
241a59d9 2185The internal variable for debugging support. The meanings of the
1fa81471
DR
2186various bits are subject to change, but currently indicate:
2187
2188=over 6
2189
2190=item 0x01
2191
2192Debug subroutine enter/exit.
2193
2194=item 0x02
2195
241a59d9
FC
2196Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2197each statement executed. Also causes saving source code lines (like
21980x400).
1fa81471
DR
2199
2200=item 0x04
2201
2202Switch off optimizations.
2203
2204=item 0x08
2205
2206Preserve more data for future interactive inspections.
2207
2208=item 0x10
2209
2210Keep info about source lines on which a subroutine is defined.
2211
2212=item 0x20
2213
2214Start with single-step on.
2215
2216=item 0x40
2217
2218Use subroutine address instead of name when reporting.
2219
2220=item 0x80
2221
2222Report C<goto &subroutine> as well.
2223
2224=item 0x100
2225
2226Provide informative "file" names for evals based on the place they were compiled.
2227
2228=item 0x200
2229
2230Provide informative names to anonymous subroutines based on the place they
2231were compiled.
2232
2233=item 0x400
2234
2235Save source code lines into C<@{"_<$filename"}>.
2236
aab47982
RS
2237=item 0x800
2238
2239When saving source, include evals that generate no subroutines.
2240
2241=item 0x1000
2242
2243When saving source, include source that did not compile.
2244
1fa81471
DR
2245=back
2246
2247Some bits may be relevant at compile-time only, some at
241a59d9 2248run-time only. This is a new mechanism and the details may change.
1fa81471
DR
2249See also L<perldebguts>.
2250
2251=item ${^TAINT}
2252X<${^TAINT}>
2253
241a59d9 2254Reflects if taint mode is on or off. 1 for on (the program was run with
1fa81471
DR
2255B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2256B<-t> or B<-TU>).
2257
2258This variable is read-only.
2259
60cf4914 2260This variable was added in Perl v5.8.0.
1fa81471 2261
f512d242
KW
2262=item ${^SAFE_LOCALES}
2263X<${^SAFE_LOCALES}>
2264
2265Reflects if safe locale operations are available to this perl (when the
2266value is 1) or not (the value is 0). This variable is always 1 if the
e9bc6d6b
KW
2267perl has been compiled without threads. It is also 1 if this perl is
2268using thread-safe locale operations. Note that an individual thread may
2269choose to use the global locale (generally unsafe) by calling
2270C<switch_to_global_locale>. This variable currently is still
2271set to 1 in such threads.
f512d242
KW
2272
2273This variable is read-only.
2274
2275This variable was added in Perl v5.28.0.
2276
1fa81471
DR
2277=item ${^UNICODE}
2278X<${^UNICODE}>
2279
241a59d9 2280Reflects certain Unicode settings of Perl. See L<perlrun>
1fa81471
DR
2281documentation for the C<-C> switch for more information about
2282the possible values.
2283
2284This variable is set during Perl startup and is thereafter read-only.
2285
60cf4914 2286This variable was added in Perl v5.8.2.
1fa81471
DR
2287
2288=item ${^UTF8CACHE}
2289X<${^UTF8CACHE}>
2290
2291This variable controls the state of the internal UTF-8 offset caching code.
22921 for on (the default), 0 for off, -1 to debug the caching code by checking
2293all its results against linear scans, and panicking on any discrepancy.
2294
94df5432
KW
2295This variable was added in Perl v5.8.9. It is subject to change or
2296removal without notice, but is currently used to avoid recalculating the
2297boundaries of multi-byte UTF-8-encoded characters.
1fa81471
DR
2298
2299=item ${^UTF8LOCALE}
2300X<${^UTF8LOCALE}>
2301
2302This variable indicates whether a UTF-8 locale was detected by perl at
241a59d9 2303startup. This information is used by perl when it's in
1fa81471
DR
2304adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2305switch); see L<perlrun> for more info on this.
2306
60cf4914 2307This variable was added in Perl v5.8.8.
1fa81471
DR
2308
2309=back
2310
b0c22438 2311=head2 Deprecated and removed variables
7f315d2e 2312
0b9346e6 2313Deprecating a variable announces the intent of the perl maintainers to
241a59d9
FC
2314eventually remove the variable from the language. It may still be
2315available despite its status. Using a deprecated variable triggers
b0c22438 2316a warning.
7f315d2e 2317
84dabc03 2318Once a variable is removed, its use triggers an error telling you
b0c22438 2319the variable is unsupported.
7f315d2e 2320
84dabc03 2321See L<perldiag> for details about error messages.
7f315d2e 2322
b0c22438 2323=over 8
7f315d2e 2324
84dabc03 2325=item $#
b7a15f05 2326X<$#>
84dabc03 2327
38e5787b 2328C<$#> was a variable that could be used to format printed numbers.
60cf4914 2329After a deprecation cycle, its magic was removed in Perl v5.10.0 and
84dabc03 2330using it now triggers a warning: C<$# is no longer supported>.
2331
2332This is not the sigil you use in front of an array name to get the
241a59d9
FC
2333last index, like C<$#array>. That's still how you get the last index
2334of an array in Perl. The two have nothing to do with each other.
84dabc03 2335
2336Deprecated in Perl 5.
2337
60cf4914 2338Removed in Perl v5.10.0.
84dabc03 2339
7f315d2e
CO
2340=item $*
2341X<$*>
2342
84dabc03 2343C<$*> was a variable that you could use to enable multiline matching.
60cf4914 2344After a deprecation cycle, its magic was removed in Perl v5.10.0.
7f315d2e 2345Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2346You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2347
b0c22438 2348Deprecated in Perl 5.
7f315d2e 2349
60cf4914 2350Removed in Perl v5.10.0.
7f315d2e 2351
84dabc03 2352=item $[
b7a15f05 2353X<$[>
84dabc03 2354
b82b06b8
FC
2355This variable stores the index of the first element in an array, and
2356of the first character in a substring. The default is 0, but you could
2357theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2358when subscripting and when evaluating the index() and substr() functions.
84dabc03 2359
b82b06b8
FC
2360As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2361directive, and cannot influence the behavior of any other file.
2362(That's why you can only assign compile-time constants to it.)
2363Its use is highly discouraged.
2364
60cf4914 2365Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
b82b06b8
FC
2366scopes in the same file, unlike other compile-time directives (such as
2367L<strict>). Using local() on it would bind its value strictly to a lexical
2368block. Now it is always lexically scoped.
2369
60cf4914 2370As of Perl v5.16.0, it is implemented by the L<arybase> module. See
b82b06b8 2371L<arybase> for more details on its behaviour.
84dabc03 2372
6b54f8ab
FC
2373Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2374effect, and always contains 0. Assigning 0 to it is permitted, but any
2375other value will produce an error.
2376
b82b06b8
FC
2377Mnemonic: [ begins subscripts.
2378
60cf4914 2379Deprecated in Perl v5.12.0.
e1dccc0d 2380
ff79a407
DIM
2381Assigning a non-zero value be fatal in Perl v5.30.0.
2382
b0c22438 2383=back
2b92dfce 2384
0b9346e6 2385=cut