3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<< <FH> >>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
118 Special package variables when using sort(), see L<perlfunc/sort>.
119 Because of this specialness $a and $b don't need to be declared
120 (using local(), use vars, or our()) even when using the strict
121 vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
122 if you want to be able to use them in the sort() comparison block
131 Contains the subpattern from the corresponding set of capturing
132 parentheses from the last pattern match, not counting patterns
133 matched in nested blocks that have been exited already. (Mnemonic:
134 like \digits.) These variables are all read-only and dynamically
135 scoped to the current BLOCK.
141 The string matched by the last successful pattern match (not counting
142 any matches hidden within a BLOCK or eval() enclosed by the current
143 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
144 and dynamically scoped to the current BLOCK.
146 The use of this variable anywhere in a program imposes a considerable
147 performance penalty on all regular expression matches. See L<BUGS>.
153 The string preceding whatever was matched by the last successful
154 pattern match (not counting any matches hidden within a BLOCK or eval
155 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
156 string.) This variable is read-only.
158 The use of this variable anywhere in a program imposes a considerable
159 performance penalty on all regular expression matches. See L<BUGS>.
165 The string following whatever was matched by the last successful
166 pattern match (not counting any matches hidden within a BLOCK or eval()
167 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
172 print "$`:$&:$'\n"; # prints abc:def:ghi
174 This variable is read-only and dynamically scoped to the current BLOCK.
176 The use of this variable anywhere in a program imposes a considerable
177 performance penalty on all regular expression matches. See L<BUGS>.
179 =item $LAST_PAREN_MATCH
183 The text matched by the last bracket of the last successful search pattern.
184 This is useful if you don't know which one of a set of alternative patterns
185 matched. For example:
187 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
189 (Mnemonic: be positive and forward looking.)
190 This variable is read-only and dynamically scoped to the current BLOCK.
194 The text matched by the used group most-recently closed (i.e. the group
195 with the rightmost closing parenthesis) of the last successful search
196 pattern. This is primarly used inside C<(?{...})> blocks for examining text
197 recently matched. For example, to effectively capture text to a variable
198 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
200 (?:(...)(?{ $var = $^N }))
202 By setting and then using C<$var> in this way relieves you from having to
203 worry about exactly which numbered set of parentheses they are.
205 This variable is dynamically scoped to the current BLOCK.
207 =item @LAST_MATCH_END
211 This array holds the offsets of the ends of the last successful
212 submatches in the currently active dynamic scope. C<$+[0]> is
213 the offset into the string of the end of the entire match. This
214 is the same value as what the C<pos> function returns when called
215 on the variable that was matched against. The I<n>th element
216 of this array holds the offset of the I<n>th submatch, so
217 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
218 past where $2 ends, and so on. You can use C<$#+> to determine
219 how many subgroups were in the last successful match. See the
220 examples given for the C<@-> variable.
222 =item $MULTILINE_MATCHING
226 Set to a non-zero integer value to do multi-line matching within a
227 string, 0 (or undefined) to tell Perl that it can assume that strings
228 contain a single line, for the purpose of optimizing pattern matches.
229 Pattern matches on strings containing multiple newlines can produce
230 confusing results when C<$*> is 0 or undefined. Default is undefined.
231 (Mnemonic: * matches multiple things.) This variable influences the
232 interpretation of only C<^> and C<$>. A literal newline can be searched
233 for even when C<$* == 0>.
235 Use of C<$*> is deprecated in modern Perl, supplanted by
236 the C</s> and C</m> modifiers on pattern matching.
238 Assigning a non-numerical value to C<$*> triggers a warning (and makes
239 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
240 makes that an implicit C<int> is applied on the value.
242 =item input_line_number HANDLE EXPR
244 =item $INPUT_LINE_NUMBER
250 The current input record number for the last file handle from which
251 you just read() (or called a C<seek> or C<tell> on). The value
252 may be different from the actual physical line number in the file,
253 depending on what notion of "line" is in effect--see C<$/> on how
254 to change that. An explicit close on a filehandle resets the line
255 number. Because C<< <> >> never does an explicit close, line
256 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
257 Consider this variable read-only: setting it does not reposition
258 the seek pointer; you'll have to do that on your own. Localizing C<$.>
259 has the effect of also localizing Perl's notion of "the last read
260 filehandle". (Mnemonic: many programs use "." to mean the current line
263 =item input_record_separator HANDLE EXPR
265 =item $INPUT_RECORD_SEPARATOR
271 The input record separator, newline by default. This
272 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
273 variable, including treating empty lines as a terminator if set to
274 the null string. (An empty line cannot contain any spaces
275 or tabs.) You may set it to a multi-character string to match a
276 multi-character terminator, or to C<undef> to read through the end
277 of file. Setting it to C<"\n\n"> means something slightly
278 different than setting to C<"">, if the file contains consecutive
279 empty lines. Setting to C<""> will treat two or more consecutive
280 empty lines as a single empty line. Setting to C<"\n\n"> will
281 blindly assume that the next input character belongs to the next
282 paragraph, even if it's a newline. (Mnemonic: / delimits
283 line boundaries when quoting poetry.)
285 undef $/; # enable "slurp" mode
286 $_ = <FH>; # whole file now here
289 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
290 better for something. :-)
292 Setting C<$/> to a reference to an integer, scalar containing an integer, or
293 scalar that's convertible to an integer will attempt to read records
294 instead of lines, with the maximum record size being the referenced
297 $/ = \32768; # or \"32768", or \$var_containing_32768
301 will read a record of no more than 32768 bytes from FILE. If you're
302 not reading from a record-oriented file (or your OS doesn't have
303 record-oriented files), then you'll likely get a full chunk of data
304 with every read. If a record is larger than the record size you've
305 set, you'll get the record back in pieces.
307 On VMS, record reads are done with the equivalent of C<sysread>,
308 so it's best not to mix record and non-record reads on the same
309 file. (This is unlikely to be a problem, because any file you'd
310 want to read in record mode is probably unusable in line mode.)
311 Non-VMS systems do normal I/O, so it's safe to mix record and
312 non-record reads of a file.
314 See also L<perlport/"Newlines">. Also see C<$.>.
316 =item autoflush HANDLE EXPR
318 =item $OUTPUT_AUTOFLUSH
322 If set to nonzero, forces a flush right away and after every write
323 or print on the currently selected output channel. Default is 0
324 (regardless of whether the channel is really buffered by the
325 system or not; C<$|> tells you only whether you've asked Perl
326 explicitly to flush after each write). STDOUT will
327 typically be line buffered if output is to the terminal and block
328 buffered otherwise. Setting this variable is useful primarily when
329 you are outputting to a pipe or socket, such as when you are running
330 a Perl program under B<rsh> and want to see the output as it's
331 happening. This has no effect on input buffering. See L<perlfunc/getc>
332 for that. (Mnemonic: when you want your pipes to be piping hot.)
334 =item output_field_separator HANDLE EXPR
336 =item $OUTPUT_FIELD_SEPARATOR
342 The output field separator for the print operator. Ordinarily the
343 print operator simply prints out its arguments without further
344 adornment. To get behavior more like B<awk>, set this variable as
345 you would set B<awk>'s OFS variable to specify what is printed
346 between fields. (Mnemonic: what is printed when there is a "," in
347 your print statement.)
349 =item output_record_separator HANDLE EXPR
351 =item $OUTPUT_RECORD_SEPARATOR
357 The output record separator for the print operator. Ordinarily the
358 print operator simply prints out its arguments as is, with no
359 trailing newline or other end-of-record string added. To get
360 behavior more like B<awk>, set this variable as you would set
361 B<awk>'s ORS variable to specify what is printed at the end of the
362 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
363 end of the print. Also, it's just like C<$/>, but it's what you
364 get "back" from Perl.)
366 =item $LIST_SEPARATOR
370 This is like C<$,> except that it applies to array and slice values
371 interpolated into a double-quoted string (or similar interpreted
372 string). Default is a space. (Mnemonic: obvious, I think.)
374 =item $SUBSCRIPT_SEPARATOR
380 The subscript separator for multidimensional array emulation. If you
381 refer to a hash element as
387 $foo{join($;, $a, $b, $c)}
391 @foo{$a,$b,$c} # a slice--note the @
395 ($foo{$a},$foo{$b},$foo{$c})
397 Default is "\034", the same as SUBSEP in B<awk>. If your
398 keys contain binary data there might not be any safe value for C<$;>.
399 (Mnemonic: comma (the syntactic subscript separator) is a
400 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
401 taken for something more important.)
403 Consider using "real" multidimensional arrays as described
410 The output format for printed numbers. This variable is a half-hearted
411 attempt to emulate B<awk>'s OFMT variable. There are times, however,
412 when B<awk> and Perl have differing notions of what counts as
413 numeric. The initial value is "%.I<n>g", where I<n> is the value
414 of the macro DBL_DIG from your system's F<float.h>. This is different from
415 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
416 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
418 Use of C<$#> is deprecated.
420 =item format_page_number HANDLE EXPR
422 =item $FORMAT_PAGE_NUMBER
426 The current page number of the currently selected output channel.
428 (Mnemonic: % is page number in B<nroff>.)
430 =item format_lines_per_page HANDLE EXPR
432 =item $FORMAT_LINES_PER_PAGE
436 The current page length (printable lines) of the currently selected
437 output channel. Default is 60.
439 (Mnemonic: = has horizontal lines.)
441 =item format_lines_left HANDLE EXPR
443 =item $FORMAT_LINES_LEFT
447 The number of lines left on the page of the currently selected output
450 (Mnemonic: lines_on_page - lines_printed.)
452 =item @LAST_MATCH_START
456 $-[0] is the offset of the start of the last successful match.
457 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
458 I<n>-th subpattern, or undef if the subpattern did not match.
460 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
461 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
462 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
463 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
464 matched subgroup in the last successful match. Contrast with
465 C<$#+>, the number of subgroups in the regular expression. Compare
468 This array holds the offsets of the beginnings of the last
469 successful submatches in the currently active dynamic scope.
470 C<$-[0]> is the offset into the string of the beginning of the
471 entire match. The I<n>th element of this array holds the offset
472 of the I<n>th submatch, so C<$+[1]> is the offset where $1
473 begins, C<$+[2]> the offset where $2 begins, and so on.
474 You can use C<$#-> to determine how many subgroups were in the
475 last successful match. Compare with the C<@+> variable.
477 After a match against some variable $var:
481 =item C<$`> is the same as C<substr($var, 0, $-[0])>
483 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
485 =item C<$'> is the same as C<substr($var, $+[0])>
487 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
489 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
491 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
495 =item format_name HANDLE EXPR
501 The name of the current report format for the currently selected output
502 channel. Default is the name of the filehandle. (Mnemonic: brother to
505 =item format_top_name HANDLE EXPR
507 =item $FORMAT_TOP_NAME
511 The name of the current top-of-page format for the currently selected
512 output channel. Default is the name of the filehandle with _TOP
513 appended. (Mnemonic: points to top of page.)
515 =item format_line_break_characters HANDLE EXPR
517 =item $FORMAT_LINE_BREAK_CHARACTERS
521 The current set of characters after which a string may be broken to
522 fill continuation fields (starting with ^) in a format. Default is
523 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
524 poetry is a part of a line.)
526 =item format_formfeed HANDLE EXPR
528 =item $FORMAT_FORMFEED
532 What formats output as a form feed. Default is \f.
538 The current value of the write() accumulator for format() lines. A format
539 contains formline() calls that put their result into C<$^A>. After
540 calling its format, write() prints out the contents of C<$^A> and empties.
541 So you never really see the contents of C<$^A> unless you call
542 formline() yourself and then look at it. See L<perlform> and
543 L<perlfunc/formline()>.
549 The status returned by the last pipe close, backtick (C<``>) command,
550 successful call to wait() or waitpid(), or from the system()
551 operator. This is just the 16-bit status word returned by the
552 wait() system call (or else is made up to look like it). Thus, the
553 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
554 C<$? & 127> gives which signal, if any, the process died from, and
555 C<$? & 128> reports whether there was a core dump. (Mnemonic:
556 similar to B<sh> and B<ksh>.)
558 Additionally, if the C<h_errno> variable is supported in C, its value
559 is returned via $? if any C<gethost*()> function fails.
561 If you have installed a signal handler for C<SIGCHLD>, the
562 value of C<$?> will usually be wrong outside that handler.
564 Inside an C<END> subroutine C<$?> contains the value that is going to be
565 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
566 change the exit status of your program. For example:
569 $? = 1 if $? == 255; # die would make it 255
572 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
573 actual VMS exit status, instead of the default emulation of POSIX
576 Also see L<Error Indicators>.
584 If used numerically, yields the current value of the C C<errno>
585 variable, with all the usual caveats. (This means that you shouldn't
586 depend on the value of C<$!> to be anything in particular unless
587 you've gotten a specific error return indicating a system error.)
588 If used an a string, yields the corresponding system error string.
589 You can assign a number to C<$!> to set I<errno> if, for instance,
590 you want C<"$!"> to return the string for error I<n>, or you want
591 to set the exit value for the die() operator. (Mnemonic: What just
594 Also see L<Error Indicators>.
596 =item $EXTENDED_OS_ERROR
600 Error information specific to the current operating system. At
601 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
602 (and for MacPerl). On all other platforms, C<$^E> is always just
605 Under VMS, C<$^E> provides the VMS status value from the last
606 system error. This is more specific information about the last
607 system error than that provided by C<$!>. This is particularly
608 important when C<$!> is set to B<EVMSERR>.
610 Under OS/2, C<$^E> is set to the error code of the last call to
611 OS/2 API either via CRT, or directly from perl.
613 Under Win32, C<$^E> always returns the last error information
614 reported by the Win32 call C<GetLastError()> which describes
615 the last error from within the Win32 API. Most Win32-specific
616 code will report errors via C<$^E>. ANSI C and Unix-like calls
617 set C<errno> and so most portable Perl code will report errors
620 Caveats mentioned in the description of C<$!> generally apply to
621 C<$^E>, also. (Mnemonic: Extra error explanation.)
623 Also see L<Error Indicators>.
629 The Perl syntax error message from the last eval() operator.
630 If $@ is the null string, the last eval() parsed and executed
631 correctly (although the operations you invoked may have failed in the
632 normal fashion). (Mnemonic: Where was the syntax error "at"?)
634 Warning messages are not collected in this variable. You can,
635 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
638 Also see L<Error Indicators>.
646 The process number of the Perl running this script. You should
647 consider this variable read-only, although it will be altered
648 across fork() calls. (Mnemonic: same as shells.)
656 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
657 if you're running setuid.) You can change both the real uid and
658 the effective uid at the same time by using POSIX::setuid().
660 =item $EFFECTIVE_USER_ID
666 The effective uid of this process. Example:
668 $< = $>; # set real to effective uid
669 ($<,$>) = ($>,$<); # swap real and effective uid
671 You can change both the effective uid and the real uid at the same
672 time by using POSIX::setuid().
674 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
675 C<< $< >> and C<< $> >> can be swapped only on machines
676 supporting setreuid().
684 The real gid of this process. If you are on a machine that supports
685 membership in multiple groups simultaneously, gives a space separated
686 list of groups you are in. The first number is the one returned by
687 getgid(), and the subsequent ones by getgroups(), one of which may be
688 the same as the first number.
690 However, a value assigned to C<$(> must be a single number used to
691 set the real gid. So the value given by C<$(> should I<not> be assigned
692 back to C<$(> without being forced numeric, such as by adding zero.
694 You can change both the real gid and the effective gid at the same
695 time by using POSIX::setgid().
697 (Mnemonic: parentheses are used to I<group> things. The real gid is the
698 group you I<left>, if you're running setgid.)
700 =item $EFFECTIVE_GROUP_ID
706 The effective gid of this process. If you are on a machine that
707 supports membership in multiple groups simultaneously, gives a space
708 separated list of groups you are in. The first number is the one
709 returned by getegid(), and the subsequent ones by getgroups(), one of
710 which may be the same as the first number.
712 Similarly, a value assigned to C<$)> must also be a space-separated
713 list of numbers. The first number sets the effective gid, and
714 the rest (if any) are passed to setgroups(). To get the effect of an
715 empty list for setgroups(), just repeat the new effective gid; that is,
716 to force an effective gid of 5 and an effectively empty setgroups()
717 list, say C< $) = "5 5" >.
719 You can change both the effective gid and the real gid at the same
720 time by using POSIX::setgid() (use only a single numeric argument).
722 (Mnemonic: parentheses are used to I<group> things. The effective gid
723 is the group that's I<right> for you, if you're running setgid.)
725 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
726 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
727 and C<$)> can be swapped only on machines supporting setregid().
733 Contains the name of the program being executed. On some operating
734 systems assigning to C<$0> modifies the argument area that the B<ps>
735 program sees. This is more useful as a way of indicating the current
736 program state than it is for hiding the program you're running.
737 (Mnemonic: same as B<sh> and B<ksh>.)
739 Note for BSD users: setting C<$0> does not completely remove "perl"
740 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
741 result in C<"perl: foobar (perl)">. This is an operating system
746 The index of the first element in an array, and of the first character
747 in a substring. Default is 0, but you could theoretically set it
748 to 1 to make Perl behave more like B<awk> (or Fortran) when
749 subscripting and when evaluating the index() and substr() functions.
750 (Mnemonic: [ begins subscripts.)
752 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
753 directive, and cannot influence the behavior of any other file.
754 Its use is highly discouraged.
758 The version + patchlevel / 1000 of the Perl interpreter. This variable
759 can be used to determine whether the Perl interpreter executing a
760 script is in the right range of versions. (Mnemonic: Is this version
761 of perl in the right bracket?) Example:
763 warn "No checksumming!\n" if $] < 3.019;
765 See also the documentation of C<use VERSION> and C<require VERSION>
766 for a convenient way to fail if the running Perl interpreter is too old.
768 The use of this variable is deprecated. The floating point representation
769 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
770 more modern representation of the Perl version that allows accurate string
777 The current value of the flag associated with the B<-c> switch.
778 Mainly of use with B<-MO=...> to allow code to alter its behavior
779 when being compiled, such as for example to AUTOLOAD at compile
780 time rather than normal, deferred loading. See L<perlcc>. Setting
781 C<$^C = 1> is similar to calling C<B::minus_c>.
787 The current value of the debugging flags. (Mnemonic: value of B<-D>
794 The maximum system file descriptor, ordinarily 2. System file
795 descriptors are passed to exec()ed processes, while higher file
796 descriptors are not. Also, during an open(), system file descriptors are
797 preserved even if the open() fails. (Ordinary file descriptors are
798 closed before the open() is attempted.) The close-on-exec
799 status of a file descriptor will be decided according to the value of
800 C<$^F> when the corresponding file, pipe, or socket was opened, not the
805 WARNING: This variable is strictly for internal use only. Its availability,
806 behavior, and contents are subject to change without notice.
808 This variable contains compile-time hints for the Perl interpreter. At the
809 end of compilation of a BLOCK the value of this variable is restored to the
810 value when the interpreter started to compile the BLOCK.
812 When perl begins to parse any block construct that provides a lexical scope
813 (e.g., eval body, required file, subroutine body, loop body, or conditional
814 block), the existing value of $^H is saved, but its value is left unchanged.
815 When the compilation of the block is completed, it regains the saved value.
816 Between the points where its value is saved and restored, code that
817 executes within BEGIN blocks is free to change the value of $^H.
819 This behavior provides the semantic of lexical scoping, and is used in,
820 for instance, the C<use strict> pragma.
822 The contents should be an integer; different bits of it are used for
823 different pragmatic flags. Here's an example:
825 sub add_100 { $^H |= 0x100 }
832 Consider what happens during execution of the BEGIN block. At this point
833 the BEGIN block has already been compiled, but the body of foo() is still
834 being compiled. The new value of $^H will therefore be visible only while
835 the body of foo() is being compiled.
837 Substitution of the above BEGIN block with:
839 BEGIN { require strict; strict->import('vars') }
841 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
842 version of the same lexical pragma:
844 BEGIN { require strict; strict->import('vars') if $condition }
848 WARNING: This variable is strictly for internal use only. Its availability,
849 behavior, and contents are subject to change without notice.
851 The %^H hash provides the same scoping semantic as $^H. This makes it
852 useful for implementation of lexically scoped pragmas.
858 The current value of the inplace-edit extension. Use C<undef> to disable
859 inplace editing. (Mnemonic: value of B<-i> switch.)
863 By default, running out of memory is an untrappable, fatal error.
864 However, if suitably built, Perl can use the contents of C<$^M>
865 as an emergency memory pool after die()ing. Suppose that your Perl
866 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
869 $^M = 'a' x (1 << 16);
871 would allocate a 64K buffer for use in an emergency. See the
872 F<INSTALL> file in the Perl distribution for information on how to
873 enable this option. To discourage casual use of this advanced
874 feature, there is no L<English|English> long name for this variable.
880 The name of the operating system under which this copy of Perl was
881 built, as determined during the configuration process. The value
882 is identical to C<$Config{'osname'}>. See also L<Config> and the
883 B<-V> command-line switch documented in L<perlrun>.
889 The internal variable for debugging support. The meanings of the
890 various bits are subject to change, but currently indicate:
896 Debug subroutine enter/exit.
900 Line-by-line debugging.
904 Switch off optimizations.
908 Preserve more data for future interactive inspections.
912 Keep info about source lines on which a subroutine is defined.
916 Start with single-step on.
920 Use subroutine address instead of name when reporting.
924 Report C<goto &subroutine> as well.
928 Provide informative "file" names for evals based on the place they were compiled.
932 Provide informative names to anonymous subroutines based on the place they
937 Some bits may be relevant at compile-time only, some at
938 run-time only. This is a new mechanism and the details may change.
940 =item $LAST_REGEXP_CODE_RESULT
944 The result of evaluation of the last successful C<(?{ code })>
945 regular expression assertion (see L<perlre>). May be written to.
947 =item $EXCEPTIONS_BEING_CAUGHT
951 Current state of the interpreter. Undefined if parsing of the current
952 module/eval is not finished (may happen in $SIG{__DIE__} and
953 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
959 The time at which the program began running, in seconds since the
960 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
961 and B<-C> filetests are based on this value.
967 The revision, version, and subversion of the Perl interpreter, represented
968 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
969 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
970 C<$^V eq v5.6.0>. Note that the characters in this string value can
971 potentially be in Unicode range.
973 This can be used to determine whether the Perl interpreter executing a
974 script is in the right range of versions. (Mnemonic: use ^V for Version
977 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
979 See the documentation of C<use VERSION> and C<require VERSION>
980 for a convenient way to fail if the running Perl interpreter is too old.
982 See also C<$]> for an older representation of the Perl version.
988 The current value of the warning switch, initially true if B<-w>
989 was used, false otherwise, but directly modifiable. (Mnemonic:
990 related to the B<-w> switch.) See also L<warnings>.
992 =item ${^WARNING_BITS}
994 The current set of warning checks enabled by the C<use warnings> pragma.
995 See the documentation of C<warnings> for more details.
997 =item ${^WIDE_SYSTEM_CALLS}
999 Global flag that enables system calls made by Perl to use wide character
1000 APIs native to the system, if available. This is currently only implemented
1001 on the Windows platform.
1003 This can also be enabled from the command line using the C<-C> switch.
1005 The initial value is typically C<0> for compatibility with Perl versions
1006 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
1007 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
1009 The C<bytes> pragma always overrides the effect of this flag in the current
1010 lexical scope. See L<bytes>.
1012 =item $EXECUTABLE_NAME
1016 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
1017 This may not be a full pathname, nor even necessarily in your path.
1021 contains the name of the current file when reading from <>.
1025 The array @ARGV contains the command-line arguments intended for
1026 the script. C<$#ARGV> is generally the number of arguments minus
1027 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1028 command name itself. See C<$0> for the command name.
1032 The array @F contains the fields of each line read in when autosplit
1033 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1034 is package-specific, and must be declared or given a full package name
1035 if not in package main when running under C<strict 'vars'>.
1039 The array @INC contains the list of places that the C<do EXPR>,
1040 C<require>, or C<use> constructs look for their library files. It
1041 initially consists of the arguments to any B<-I> command-line
1042 switches, followed by the default Perl library, probably
1043 F</usr/local/lib/perl>, followed by ".", to represent the current
1044 directory. If you need to modify this at runtime, you should use
1045 the C<use lib> pragma to get the machine-dependent library properly
1048 use lib '/mypath/libdir/';
1053 Within a subroutine the array @_ contains the parameters passed to that
1054 subroutine. See L<perlsub>.
1058 The hash %INC contains entries for each filename included via the
1059 C<do>, C<require>, or C<use> operators. The key is the filename
1060 you specified (with module names converted to pathnames), and the
1061 value is the location of the file found. The C<require>
1062 operator uses this hash to determine whether a particular file has
1063 already been included.
1069 The hash %ENV contains your current environment. Setting a
1070 value in C<ENV> changes the environment for any child processes
1071 you subsequently fork() off.
1077 The hash %SIG contains signal handlers for signals. For example:
1079 sub handler { # 1st argument is signal name
1081 print "Caught a SIG$sig--shutting down\n";
1086 $SIG{'INT'} = \&handler;
1087 $SIG{'QUIT'} = \&handler;
1089 $SIG{'INT'} = 'DEFAULT'; # restore default action
1090 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1092 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1093 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1096 Here are some other examples:
1098 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1099 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1100 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1101 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1103 Be sure not to use a bareword as the name of a signal handler,
1104 lest you inadvertently call it.
1106 If your system has the sigaction() function then signal handlers are
1107 installed using it. This means you get reliable signal handling. If
1108 your system has the SA_RESTART flag it is used when signals handlers are
1109 installed. This means that system calls for which restarting is supported
1110 continue rather than returning when a signal arrives. If you want your
1111 system calls to be interrupted by signal delivery then do something like
1114 use POSIX ':signal_h';
1117 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1118 or die "Error setting SIGALRM handler: $!\n";
1122 Certain internal hooks can be also set using the %SIG hash. The
1123 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1124 about to be printed. The warning message is passed as the first
1125 argument. The presence of a __WARN__ hook causes the ordinary printing
1126 of warnings to STDERR to be suppressed. You can use this to save warnings
1127 in a variable, or turn warnings into fatal errors, like this:
1129 local $SIG{__WARN__} = sub { die $_[0] };
1132 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1133 is about to be thrown. The error message is passed as the first
1134 argument. When a __DIE__ hook routine returns, the exception
1135 processing continues as it would have in the absence of the hook,
1136 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1137 The C<__DIE__> handler is explicitly disabled during the call, so that you
1138 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1140 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1141 even inside an eval(). Do not use this to rewrite a pending exception
1142 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1143 This strange action at a distance may be fixed in a future release
1144 so that C<$SIG{__DIE__}> is only called if your program is about
1145 to exit, as was the original intent. Any other use is deprecated.
1147 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1148 they may be called to report (probable) errors found by the parser.
1149 In such a case the parser may be in inconsistent state, so any
1150 attempt to evaluate Perl code from such a handler will probably
1151 result in a segfault. This means that warnings or errors that
1152 result from parsing Perl should be used with extreme caution, like
1155 require Carp if defined $^S;
1156 Carp::confess("Something wrong") if defined &Carp::confess;
1157 die "Something wrong, but could not load Carp to give backtrace...
1158 To see backtrace try starting Perl with -MCarp switch";
1160 Here the first line will load Carp I<unless> it is the parser who
1161 called the handler. The second line will print backtrace and die if
1162 Carp was available. The third line will be executed only if Carp was
1165 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1166 L<warnings> for additional information.
1170 =head2 Error Indicators
1172 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1173 about different types of error conditions that may appear during
1174 execution of a Perl program. The variables are shown ordered by
1175 the "distance" between the subsystem which reported the error and
1176 the Perl process. They correspond to errors detected by the Perl
1177 interpreter, C library, operating system, or an external program,
1180 To illustrate the differences between these variables, consider the
1181 following Perl expression, which uses a single-quoted string:
1184 open PIPE, "/cdrom/install |";
1186 close PIPE or die "bad pipe: $?, $!";
1189 After execution of this statement all 4 variables may have been set.
1191 C<$@> is set if the string to be C<eval>-ed did not compile (this
1192 may happen if C<open> or C<close> were imported with bad prototypes),
1193 or if Perl code executed during evaluation die()d . In these cases
1194 the value of $@ is the compile error, or the argument to C<die>
1195 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1198 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1199 and C<close> are translated to calls in the C run-time library and
1200 thence to the operating system kernel. C<$!> is set to the C library's
1201 C<errno> if one of these calls fails.
1203 Under a few operating systems, C<$^E> may contain a more verbose
1204 error indicator, such as in this case, "CDROM tray not closed."
1205 Systems that do not support extended error messages leave C<$^E>
1208 Finally, C<$?> may be set to non-0 value if the external program
1209 F</cdrom/install> fails. The upper eight bits reflect specific
1210 error conditions encountered by the program (the program's exit()
1211 value). The lower eight bits reflect mode of failure, like signal
1212 death and core dump information See wait(2) for details. In
1213 contrast to C<$!> and C<$^E>, which are set only if error condition
1214 is detected, the variable C<$?> is set on each C<wait> or pipe
1215 C<close>, overwriting the old value. This is more like C<$@>, which
1216 on every eval() is always set on failure and cleared on success.
1218 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1221 =head2 Technical Note on the Syntax of Variable Names
1223 Variable names in Perl can have several formats. Usually, they
1224 must begin with a letter or underscore, in which case they can be
1225 arbitrarily long (up to an internal limit of 251 characters) and
1226 may contain letters, digits, underscores, or the special sequence
1227 C<::> or C<'>. In this case, the part before the last C<::> or
1228 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1230 Perl variable names may also be a sequence of digits or a single
1231 punctuation or control character. These names are all reserved for
1232 special uses by Perl; for example, the all-digits names are used
1233 to hold data captured by backreferences after a regular expression
1234 match. Perl has a special syntax for the single-control-character
1235 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1236 character. For example, the notation C<$^W> (dollar-sign caret
1237 C<W>) is the scalar variable whose name is the single character
1238 control-C<W>. This is better than typing a literal control-C<W>
1241 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1242 strings that begin with control characters (or better yet, a caret).
1243 These variables must be written in the form C<${^Foo}>; the braces
1244 are not optional. C<${^Foo}> denotes the scalar variable whose
1245 name is a control-C<F> followed by two C<o>'s. These variables are
1246 reserved for future special uses by Perl, except for the ones that
1247 begin with C<^_> (control-underscore or caret-underscore). No
1248 control-character name that begins with C<^_> will acquire a special
1249 meaning in any future version of Perl; such names may therefore be
1250 used safely in programs. C<$^_> itself, however, I<is> reserved.
1252 Perl identifiers that begin with digits, control characters, or
1253 punctuation characters are exempt from the effects of the C<package>
1254 declaration and are always forced to be in package C<main>. A few
1255 other names are also exempt:
1263 In particular, the new special C<${^_XYZ}> variables are always taken
1264 to be in package C<main>, regardless of any C<package> declarations
1269 Due to an unfortunate accident of Perl's implementation, C<use
1270 English> imposes a considerable performance penalty on all regular
1271 expression matches in a program, regardless of whether they occur
1272 in the scope of C<use English>. For that reason, saying C<use
1273 English> in libraries is strongly discouraged. See the
1274 Devel::SawAmpersand module documentation from CPAN
1275 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1276 for more information.
1278 Having to even think about the C<$^S> variable in your exception
1279 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1280 invites grievous and difficult to track down errors. Avoid it
1281 and use an C<END{}> or CORE::GLOBAL::die override instead.