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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
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10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
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13
14 use English;
15
16at the top of your program. This will alias all the short names to the
5a964f20 17long names in the current package. Some even have medium names,
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18generally borrowed from B<awk>.
19
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20If you don't mind the performance hit, variables that depend on the
21currently selected filehandle may instead be set by calling an
22appropriate object method on the IO::Handle object. (Summary lines
23below for this contain the word HANDLE.) First you must say
a0d0e21e 24
19799a22 25 use IO::Handle;
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26
27after which you may use either
28
29 method HANDLE EXPR
30
5a964f20 31or more safely,
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32
33 HANDLE->method(EXPR)
34
14218588 35Each method returns the old value of the IO::Handle attribute.
a0d0e21e 36The methods each take an optional EXPR, which if supplied specifies the
19799a22 37new value for the IO::Handle attribute in question. If not supplied,
14218588 38most methods do nothing to the current value--except for
a0d0e21e 39autoflush(), which will assume a 1 for you, just to be different.
14218588 40Because loading in the IO::Handle class is an expensive operation, you should
19799a22 41learn how to use the regular built-in variables.
a0d0e21e 42
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43A few of these variables are considered "read-only". This means that if
44you try to assign to this variable, either directly or indirectly through
45a reference, you'll raise a run-time exception.
a0d0e21e 46
fb73857a 47The following list is ordered by scalar variables first, then the
87275199 48arrays, then the hashes.
fb73857a 49
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50=over 8
51
52=item $ARG
53
54=item $_
55
56The default input and pattern-searching space. The following pairs are
57equivalent:
58
19799a22 59 while (<>) {...} # equivalent only in while!
54310121 60 while (defined($_ = <>)) {...}
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61
62 /^Subject:/
63 $_ =~ /^Subject:/
64
65 tr/a-z/A-Z/
66 $_ =~ tr/a-z/A-Z/
67
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68 chomp
69 chomp($_)
a0d0e21e 70
54310121 71Here are the places where Perl will assume $_ even if you
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72don't use it:
73
74=over 3
75
76=item *
77
78Various unary functions, including functions like ord() and int(), as well
79as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
80STDIN.
81
82=item *
83
84Various list functions like print() and unlink().
85
86=item *
87
88The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89without an C<=~> operator.
90
54310121 91=item *
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92
93The default iterator variable in a C<foreach> loop if no other
94variable is supplied.
95
54310121 96=item *
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97
98The implicit iterator variable in the grep() and map() functions.
99
54310121 100=item *
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101
102The default place to put an input record when a C<E<lt>FHE<gt>>
103operation's result is tested by itself as the sole criterion of a C<while>
14218588 104test. Outside a C<while> test, this will not happen.
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105
106=back
107
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108(Mnemonic: underline is understood in certain operations.)
109
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110=back
111
112=over 8
113
5a964f20 114=item $E<lt>I<digits>E<gt>
a0d0e21e 115
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116Contains the subpattern from the corresponding set of capturing
117parentheses from the last pattern match, not counting patterns
118matched in nested blocks that have been exited already. (Mnemonic:
119like \digits.) These variables are all read-only and dynamically
120scoped to the current BLOCK.
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121
122=item $MATCH
123
124=item $&
125
126The string matched by the last successful pattern match (not counting
127any matches hidden within a BLOCK or eval() enclosed by the current
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128BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
129and dynamically scoped to the current BLOCK.
a0d0e21e 130
19ddd453 131The use of this variable anywhere in a program imposes a considerable
19799a22 132performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 133
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134=item $PREMATCH
135
136=item $`
137
138The string preceding whatever was matched by the last successful
139pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 140enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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141string.) This variable is read-only.
142
19ddd453 143The use of this variable anywhere in a program imposes a considerable
19799a22 144performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 145
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146=item $POSTMATCH
147
148=item $'
149
150The string following whatever was matched by the last successful
151pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 152enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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153string.) Example:
154
155 $_ = 'abcdefghi';
156 /def/;
157 print "$`:$&:$'\n"; # prints abc:def:ghi
158
19799a22 159This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 160
19ddd453 161The use of this variable anywhere in a program imposes a considerable
19799a22 162performance penalty on all regular expression matches. See L<BUGS>.
19ddd453 163
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164=item $LAST_PAREN_MATCH
165
166=item $+
167
168The last bracket matched by the last search pattern. This is useful if
19799a22 169you don't know which one of a set of alternative patterns matched. For
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170example:
171
172 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
173
174(Mnemonic: be positive and forward looking.)
19799a22 175This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 176
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177=item @+
178
19799a22 179$+[0] is the offset of the end of the last successful match.
6cef1e77 180C<$+[>I<n>C<]> is the offset of the end of the substring matched by
8f580fb8 181I<n>-th subpattern, or undef if the subpattern did not match.
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182
183Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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184$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
185$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
186C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number
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187of subgroups in the last successful match. Contrast with
188C<$#->, the last I<matched> subgroup. Compare with C<@->.
6cef1e77 189
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190=item $MULTILINE_MATCHING
191
192=item $*
193
4a6725af 194Set to 1 to do multi-line matching within a string, 0 to tell Perl
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195that it can assume that strings contain a single line, for the purpose
196of optimizing pattern matches. Pattern matches on strings containing
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197multiple newlines can produce confusing results when C<$*> is 0. Default
198is 0. (Mnemonic: * matches multiple things.) This variable
199influences the interpretation of only C<^> and C<$>. A literal newline can
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200be searched for even when C<$* == 0>.
201
19799a22 202Use of C<$*> is deprecated in modern Perl, supplanted by
5a964f20 203the C</s> and C</m> modifiers on pattern matching.
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204
205=item input_line_number HANDLE EXPR
206
207=item $INPUT_LINE_NUMBER
208
209=item $NR
210
211=item $.
212
19799a22 213The current input record number for the last file handle from which
14218588 214you just read() (or called a C<seek> or C<tell> on). The value
883faa13 215may be different from the actual physical line number in the file,
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216depending on what notion of "line" is in effect--see C<$/> on how
217to change that. An explicit close on a filehandle resets the line
218number. Because C<E<lt>E<gt>> never does an explicit close, line
219numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
220Consider this variable read-only: setting it does not reposition
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221the seek pointer; you'll have to do that on your own. Localizing C<$.>
222has the effect of also localizing Perl's notion of "the last read
223filehandle". (Mnemonic: many programs use "." to mean the current line
224number.)
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225
226=item input_record_separator HANDLE EXPR
227
228=item $INPUT_RECORD_SEPARATOR
229
230=item $RS
231
232=item $/
233
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234The input record separator, newline by default. This
235influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 236variable, including treating empty lines as a terminator if set to
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237the null string. (An empty line cannot contain any spaces
238or tabs.) You may set it to a multi-character string to match a
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239multi-character terminator, or to C<undef> to read through the end
240of file. Setting it to C<"\n\n"> means something slightly
241different than setting to C<"">, if the file contains consecutive
242empty lines. Setting to C<""> will treat two or more consecutive
243empty lines as a single empty line. Setting to C<"\n\n"> will
244blindly assume that the next input character belongs to the next
14218588 245paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 246line boundaries when quoting poetry.)
a0d0e21e 247
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248 undef $/; # enable "slurp" mode
249 $_ = <FH>; # whole file now here
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250 s/\n[ \t]+/ /g;
251
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252Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
253better for something. :-)
68dc0745 254
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255Setting C<$/> to a reference to an integer, scalar containing an integer, or
256scalar that's convertible to an integer will attempt to read records
5b2b9c68 257instead of lines, with the maximum record size being the referenced
19799a22 258integer. So this:
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259
260 $/ = \32768; # or \"32768", or \$var_containing_32768
261 open(FILE, $myfile);
262 $_ = <FILE>;
263
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264will read a record of no more than 32768 bytes from FILE. If you're
265not reading from a record-oriented file (or your OS doesn't have
266record-oriented files), then you'll likely get a full chunk of data
267with every read. If a record is larger than the record size you've
268set, you'll get the record back in pieces.
5b2b9c68 269
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270On VMS, record reads are done with the equivalent of C<sysread>,
271so it's best not to mix record and non-record reads on the same
272file. (This is unlikely to be a problem, because any file you'd
83763826 273want to read in record mode is probably unusable in line mode.)
14218588 274Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 275non-record reads of a file.
5b2b9c68 276
14218588 277See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 278
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279=item autoflush HANDLE EXPR
280
281=item $OUTPUT_AUTOFLUSH
282
283=item $|
284
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285If set to nonzero, forces a flush right away and after every write
286or print on the currently selected output channel. Default is 0
14218588 287(regardless of whether the channel is really buffered by the
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288system or not; C<$|> tells you only whether you've asked Perl
289explicitly to flush after each write). STDOUT will
290typically be line buffered if output is to the terminal and block
291buffered otherwise. Setting this variable is useful primarily when
292you are outputting to a pipe or socket, such as when you are running
293a Perl program under B<rsh> and want to see the output as it's
294happening. This has no effect on input buffering. See L<perlfunc/getc>
295for that. (Mnemonic: when you want your pipes to be piping hot.)
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296
297=item output_field_separator HANDLE EXPR
298
299=item $OUTPUT_FIELD_SEPARATOR
300
301=item $OFS
302
303=item $,
304
305The output field separator for the print operator. Ordinarily the
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306print operator simply prints out its arguments without further
307adornment. To get behavior more like B<awk>, set this variable as
308you would set B<awk>'s OFS variable to specify what is printed
309between fields. (Mnemonic: what is printed when there is a "," in
310your print statement.)
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311
312=item output_record_separator HANDLE EXPR
313
314=item $OUTPUT_RECORD_SEPARATOR
315
316=item $ORS
317
318=item $\
319
320The output record separator for the print operator. Ordinarily the
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321print operator simply prints out its arguments as is, with no
322trailing newline or other end-of-record string added. To get
323behavior more like B<awk>, set this variable as you would set
324B<awk>'s ORS variable to specify what is printed at the end of the
325print. (Mnemonic: you set C<$\> instead of adding "\n" at the
326end of the print. Also, it's just like C<$/>, but it's what you
327get "back" from Perl.)
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328
329=item $LIST_SEPARATOR
330
331=item $"
332
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333This is like C<$,> except that it applies to array and slice values
334interpolated into a double-quoted string (or similar interpreted
335string). Default is a space. (Mnemonic: obvious, I think.)
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336
337=item $SUBSCRIPT_SEPARATOR
338
339=item $SUBSEP
340
341=item $;
342
54310121 343The subscript separator for multidimensional array emulation. If you
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344refer to a hash element as
345
346 $foo{$a,$b,$c}
347
348it really means
349
350 $foo{join($;, $a, $b, $c)}
351
352But don't put
353
354 @foo{$a,$b,$c} # a slice--note the @
355
356which means
357
358 ($foo{$a},$foo{$b},$foo{$c})
359
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360Default is "\034", the same as SUBSEP in B<awk>. If your
361keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 362(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 363semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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364taken for something more important.)
365
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366Consider using "real" multidimensional arrays as described
367in L<perllol>.
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368
369=item $OFMT
370
371=item $#
372
373The output format for printed numbers. This variable is a half-hearted
374attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 375when B<awk> and Perl have differing notions of what counts as
19799a22 376numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 377of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 378B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 379explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 380
19799a22 381Use of C<$#> is deprecated.
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382
383=item format_page_number HANDLE EXPR
384
385=item $FORMAT_PAGE_NUMBER
386
387=item $%
388
389The current page number of the currently selected output channel.
19799a22 390Used with formats.
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391(Mnemonic: % is page number in B<nroff>.)
392
393=item format_lines_per_page HANDLE EXPR
394
395=item $FORMAT_LINES_PER_PAGE
396
397=item $=
398
399The current page length (printable lines) of the currently selected
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400output channel. Default is 60.
401Used with formats.
402(Mnemonic: = has horizontal lines.)
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403
404=item format_lines_left HANDLE EXPR
405
406=item $FORMAT_LINES_LEFT
407
408=item $-
409
410The number of lines left on the page of the currently selected output
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411channel.
412Used with formats.
413(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 414
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415=item @-
416
19799a22 417$-[0] is the offset of the start of the last successful match.
6cef1e77 418C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 419I<n>-th subpattern, or undef if the subpattern did not match.
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420
421Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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422$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
423$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
424C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
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425matched subgroup in the last successful match. Contrast with
426C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 427with C<@+>.
6cef1e77 428
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429=item format_name HANDLE EXPR
430
431=item $FORMAT_NAME
432
433=item $~
434
435The name of the current report format for the currently selected output
14218588 436channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 437C<$^>.)
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438
439=item format_top_name HANDLE EXPR
440
441=item $FORMAT_TOP_NAME
442
443=item $^
444
445The name of the current top-of-page format for the currently selected
14218588 446output channel. Default is the name of the filehandle with _TOP
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447appended. (Mnemonic: points to top of page.)
448
449=item format_line_break_characters HANDLE EXPR
450
451=item $FORMAT_LINE_BREAK_CHARACTERS
452
453=item $:
454
455The current set of characters after which a string may be broken to
54310121 456fill continuation fields (starting with ^) in a format. Default is
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457S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
458poetry is a part of a line.)
459
460=item format_formfeed HANDLE EXPR
461
462=item $FORMAT_FORMFEED
463
464=item $^L
465
14218588 466What formats output as a form feed. Default is \f.
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467
468=item $ACCUMULATOR
469
470=item $^A
471
472The current value of the write() accumulator for format() lines. A format
19799a22 473contains formline() calls that put their result into C<$^A>. After
a0d0e21e 474calling its format, write() prints out the contents of C<$^A> and empties.
14218588 475So you never really see the contents of C<$^A> unless you call
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476formline() yourself and then look at it. See L<perlform> and
477L<perlfunc/formline()>.
478
479=item $CHILD_ERROR
480
481=item $?
482
54310121 483The status returned by the last pipe close, backtick (C<``>) command,
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484successful call to wait() or waitpid(), or from the system()
485operator. This is just the 16-bit status word returned by the
486wait() system call (or else is made up to look like it). Thus, the
14218588 487exit value of the subprocess is really (C<$? E<gt>E<gt> 8>), and
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488C<$? & 127> gives which signal, if any, the process died from, and
489C<$? & 128> reports whether there was a core dump. (Mnemonic:
490similar to B<sh> and B<ksh>.)
a0d0e21e 491
7b8d334a 492Additionally, if the C<h_errno> variable is supported in C, its value
14218588 493is returned via $? if any C<gethost*()> function fails.
7b8d334a 494
19799a22 495If you have installed a signal handler for C<SIGCHLD>, the
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496value of C<$?> will usually be wrong outside that handler.
497
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498Inside an C<END> subroutine C<$?> contains the value that is going to be
499given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
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500change the exit status of your program. For example:
501
502 END {
503 $? = 1 if $? == 255; # die would make it 255
504 }
a8f8344d 505
aa689395 506Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
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507actual VMS exit status, instead of the default emulation of POSIX
508status.
f86702cc 509
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510Also see L<Error Indicators>.
511
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512=item $OS_ERROR
513
514=item $ERRNO
515
516=item $!
517
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518If used numerically, yields the current value of the C C<errno>
519variable, with all the usual caveats. (This means that you shouldn't
520depend on the value of C<$!> to be anything in particular unless
521you've gotten a specific error return indicating a system error.)
522If used an a string, yields the corresponding system error string.
523You can assign a number to C<$!> to set I<errno> if, for instance,
524you want C<"$!"> to return the string for error I<n>, or you want
525to set the exit value for the die() operator. (Mnemonic: What just
526went bang?)
a0d0e21e 527
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528Also see L<Error Indicators>.
529
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530=item $EXTENDED_OS_ERROR
531
532=item $^E
533
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534Error information specific to the current operating system. At
535the moment, this differs from C<$!> under only VMS, OS/2, and Win32
536(and for MacPerl). On all other platforms, C<$^E> is always just
537the same as C<$!>.
538
539Under VMS, C<$^E> provides the VMS status value from the last
540system error. This is more specific information about the last
541system error than that provided by C<$!>. This is particularly
d516a115 542important when C<$!> is set to B<EVMSERR>.
22fae026 543
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544Under OS/2, C<$^E> is set to the error code of the last call to
545OS/2 API either via CRT, or directly from perl.
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546
547Under Win32, C<$^E> always returns the last error information
548reported by the Win32 call C<GetLastError()> which describes
549the last error from within the Win32 API. Most Win32-specific
19799a22 550code will report errors via C<$^E>. ANSI C and Unix-like calls
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551set C<errno> and so most portable Perl code will report errors
552via C<$!>.
553
554Caveats mentioned in the description of C<$!> generally apply to
555C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 556
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557Also see L<Error Indicators>.
558
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559=item $EVAL_ERROR
560
561=item $@
562
19799a22 563The Perl syntax error message from the last eval() operator. If null, the
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564last eval() parsed and executed correctly (although the operations you
565invoked may have failed in the normal fashion). (Mnemonic: Where was
566the syntax error "at"?)
567
19799a22 568Warning messages are not collected in this variable. You can,
a8f8344d 569however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 570as described below.
748a9306 571
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572Also see L<Error Indicators>.
573
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574=item $PROCESS_ID
575
576=item $PID
577
578=item $$
579
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580The process number of the Perl running this script. You should
581consider this variable read-only, although it will be altered
582across fork() calls. (Mnemonic: same as shells.)
a0d0e21e
LW
583
584=item $REAL_USER_ID
585
586=item $UID
587
588=item $<
589
19799a22 590The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a0d0e21e
LW
591if you're running setuid.)
592
593=item $EFFECTIVE_USER_ID
594
595=item $EUID
596
597=item $>
598
599The effective uid of this process. Example:
600
601 $< = $>; # set real to effective uid
602 ($<,$>) = ($>,$<); # swap real and effective uid
603
19799a22 604(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
14218588 605C<$E<lt>> and C<$E<gt>> can be swapped only on machines
8cc95fdb 606supporting setreuid().
a0d0e21e
LW
607
608=item $REAL_GROUP_ID
609
610=item $GID
611
612=item $(
613
614The real gid of this process. If you are on a machine that supports
615membership in multiple groups simultaneously, gives a space separated
616list of groups you are in. The first number is the one returned by
617getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
618the same as the first number.
619
19799a22
GS
620However, a value assigned to C<$(> must be a single number used to
621set the real gid. So the value given by C<$(> should I<not> be assigned
622back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 623
19799a22
GS
624(Mnemonic: parentheses are used to I<group> things. The real gid is the
625group you I<left>, if you're running setgid.)
a0d0e21e
LW
626
627=item $EFFECTIVE_GROUP_ID
628
629=item $EGID
630
631=item $)
632
633The effective gid of this process. If you are on a machine that
634supports membership in multiple groups simultaneously, gives a space
635separated list of groups you are in. The first number is the one
636returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
637which may be the same as the first number.
638
19799a22 639Similarly, a value assigned to C<$)> must also be a space-separated
14218588 640list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
641the rest (if any) are passed to setgroups(). To get the effect of an
642empty list for setgroups(), just repeat the new effective gid; that is,
643to force an effective gid of 5 and an effectively empty setgroups()
644list, say C< $) = "5 5" >.
645
19799a22
GS
646(Mnemonic: parentheses are used to I<group> things. The effective gid
647is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 648
14218588 649C<$E<lt>>, C<$E<gt>>, C<$(> and C<$)> can be set only on
19799a22
GS
650machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
651and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
652
653=item $PROGRAM_NAME
654
655=item $0
656
19799a22
GS
657Contains the name of the program being executed. On some operating
658systems assigning to C<$0> modifies the argument area that the B<ps>
659program sees. This is more useful as a way of indicating the current
660program state than it is for hiding the program you're running.
a0d0e21e
LW
661(Mnemonic: same as B<sh> and B<ksh>.)
662
663=item $[
664
665The index of the first element in an array, and of the first character
19799a22
GS
666in a substring. Default is 0, but you could theoretically set it
667to 1 to make Perl behave more like B<awk> (or Fortran) when
668subscripting and when evaluating the index() and substr() functions.
669(Mnemonic: [ begins subscripts.)
a0d0e21e 670
19799a22
GS
671As of release 5 of Perl, assignment to C<$[> is treated as a compiler
672directive, and cannot influence the behavior of any other file.
673Its use is highly discouraged.
a0d0e21e
LW
674
675=item $PERL_VERSION
676
677=item $]
678
54310121
PP
679The version + patchlevel / 1000 of the Perl interpreter. This variable
680can be used to determine whether the Perl interpreter executing a
681script is in the right range of versions. (Mnemonic: Is this version
682of perl in the right bracket?) Example:
a0d0e21e
LW
683
684 warn "No checksumming!\n" if $] < 3.019;
685
54310121 686See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 687for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 688
305aace0
NIS
689=item $COMPILING
690
691=item $^C
692
19799a22
GS
693The current value of the flag associated with the B<-c> switch.
694Mainly of use with B<-MO=...> to allow code to alter its behavior
695when being compiled, such as for example to AUTOLOAD at compile
696time rather than normal, deferred loading. See L<perlcc>. Setting
697C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 698
a0d0e21e
LW
699=item $DEBUGGING
700
701=item $^D
702
703The current value of the debugging flags. (Mnemonic: value of B<-D>
704switch.)
705
706=item $SYSTEM_FD_MAX
707
708=item $^F
709
710The maximum system file descriptor, ordinarily 2. System file
711descriptors are passed to exec()ed processes, while higher file
712descriptors are not. Also, during an open(), system file descriptors are
713preserved even if the open() fails. (Ordinary file descriptors are
19799a22 714closed before the open() is attempted.) The close-on-exec
a0d0e21e 715status of a file descriptor will be decided according to the value of
4771b018 716C<$^F> when the open() or pipe() was called, not the time of the exec().
a0d0e21e 717
6e2995f4
PP
718=item $^H
719
0462a1ab
GS
720WARNING: This variable is strictly for internal use only. Its availability,
721behavior, and contents are subject to change without notice.
722
723This variable contains compile-time hints for the Perl interpreter. At the
724end of compilation of a BLOCK the value of this variable is restored to the
725value when the interpreter started to compile the BLOCK.
726
727When perl begins to parse any block construct that provides a lexical scope
728(e.g., eval body, required file, subroutine body, loop body, or conditional
729block), the existing value of $^H is saved, but its value is left unchanged.
730When the compilation of the block is completed, it regains the saved value.
731Between the points where its value is saved and restored, code that
732executes within BEGIN blocks is free to change the value of $^H.
733
734This behavior provides the semantic of lexical scoping, and is used in,
735for instance, the C<use strict> pragma.
736
737The contents should be an integer; different bits of it are used for
738different pragmatic flags. Here's an example:
739
740 sub add_100 { $^H |= 0x100 }
741
742 sub foo {
743 BEGIN { add_100() }
744 bar->baz($boon);
745 }
746
747Consider what happens during execution of the BEGIN block. At this point
748the BEGIN block has already been compiled, but the body of foo() is still
749being compiled. The new value of $^H will therefore be visible only while
750the body of foo() is being compiled.
751
752Substitution of the above BEGIN block with:
753
754 BEGIN { require strict; strict->import('vars') }
755
756demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
757version of the same lexical pragma:
758
759 BEGIN { require strict; strict->import('vars') if $condition }
760
761=item %^H
762
763WARNING: This variable is strictly for internal use only. Its availability,
764behavior, and contents are subject to change without notice.
765
766The %^H hash provides the same scoping semantic as $^H. This makes it
767useful for implementation of lexically scoped pragmas.
6e2995f4 768
a0d0e21e
LW
769=item $INPLACE_EDIT
770
771=item $^I
772
773The current value of the inplace-edit extension. Use C<undef> to disable
774inplace editing. (Mnemonic: value of B<-i> switch.)
775
fb73857a
PP
776=item $^M
777
19799a22
GS
778By default, running out of memory is an untrappable, fatal error.
779However, if suitably built, Perl can use the contents of C<$^M>
780as an emergency memory pool after die()ing. Suppose that your Perl
781were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
782Then
fb73857a 783
19799a22 784 $^M = 'a' x (1 << 16);
fb73857a 785
19799a22
GS
786would allocate a 64K buffer for use when in emergency. See the
787F<INSTALL> file in the Perl distribution for information on how to
788enable this option. To discourage casual use of this advanced
789feature, there is no L<English> long name for this variable.
fb73857a 790
5c055ba3 791=item $OSNAME
6e2995f4 792
5c055ba3
PP
793=item $^O
794
795The name of the operating system under which this copy of Perl was
796built, as determined during the configuration process. The value
19799a22
GS
797is identical to C<$Config{'osname'}>. See also L<Config> and the
798B<-V> command-line switch documented in L<perlrun>.
5c055ba3 799
a0d0e21e
LW
800=item $PERLDB
801
802=item $^P
803
19799a22
GS
804The internal variable for debugging support. The meanings of the
805various bits are subject to change, but currently indicate:
84902520
TB
806
807=over 6
808
809=item 0x01
810
811Debug subroutine enter/exit.
812
813=item 0x02
814
815Line-by-line debugging.
816
817=item 0x04
818
819Switch off optimizations.
820
821=item 0x08
822
823Preserve more data for future interactive inspections.
824
825=item 0x10
826
827Keep info about source lines on which a subroutine is defined.
828
829=item 0x20
830
831Start with single-step on.
832
833=back
834
19799a22
GS
835Some bits may be relevant at compile-time only, some at
836run-time only. This is a new mechanism and the details may change.
a0d0e21e 837
b9ac3b5b
GS
838=item $^R
839
19799a22
GS
840The result of evaluation of the last successful C<(?{ code })>
841regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 842
fb73857a
PP
843=item $^S
844
845Current state of the interpreter. Undefined if parsing of the current
846module/eval is not finished (may happen in $SIG{__DIE__} and
19799a22 847$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
fb73857a 848
a0d0e21e
LW
849=item $BASETIME
850
851=item $^T
852
19799a22 853The time at which the program began running, in seconds since the
5f05dabc 854epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 855and B<-C> filetests are based on this value.
a0d0e21e
LW
856
857=item $WARNING
858
859=item $^W
860
19799a22
GS
861The current value of the warning switch, initially true if B<-w>
862was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
863related to the B<-w> switch.) See also L<warnings>.
864
865=item ${^Warnings}
866
867The current set of warning checks enabled by the C<use warnings> pragma.
868See the documentation of C<warnings> for more details.
a0d0e21e
LW
869
870=item $EXECUTABLE_NAME
871
872=item $^X
873
874The name that the Perl binary itself was executed as, from C's C<argv[0]>.
19799a22 875This may not be a full pathname, nor even necessarily in your path.
a0d0e21e
LW
876
877=item $ARGV
878
a8f8344d 879contains the name of the current file when reading from E<lt>E<gt>.
a0d0e21e
LW
880
881=item @ARGV
882
19799a22 883The array @ARGV contains the command-line arguments intended for
14218588 884the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
885one, because C<$ARGV[0]> is the first argument, I<not> the program's
886command name itself. See C<$0> for the command name.
a0d0e21e
LW
887
888=item @INC
889
19799a22
GS
890The array @INC contains the list of places that the C<do EXPR>,
891C<require>, or C<use> constructs look for their library files. It
892initially consists of the arguments to any B<-I> command-line
893switches, followed by the default Perl library, probably
894F</usr/local/lib/perl>, followed by ".", to represent the current
895directory. If you need to modify this at runtime, you should use
896the C<use lib> pragma to get the machine-dependent library properly
897loaded also:
a0d0e21e 898
cb1a09d0
AD
899 use lib '/mypath/libdir/';
900 use SomeMod;
303f2f76 901
fb73857a
PP
902=item @_
903
904Within a subroutine the array @_ contains the parameters passed to that
19799a22 905subroutine. See L<perlsub>.
fb73857a 906
a0d0e21e
LW
907=item %INC
908
19799a22
GS
909The hash %INC contains entries for each filename included via the
910C<do>, C<require>, or C<use> operators. The key is the filename
911you specified (with module names converted to pathnames), and the
14218588 912value is the location of the file found. The C<require>
87275199 913operator uses this hash to determine whether a particular file has
19799a22 914already been included.
a0d0e21e 915
b687b08b
TC
916=item %ENV
917
918=item $ENV{expr}
a0d0e21e
LW
919
920The hash %ENV contains your current environment. Setting a
19799a22
GS
921value in C<ENV> changes the environment for any child processes
922you subsequently fork() off.
a0d0e21e 923
b687b08b
TC
924=item %SIG
925
926=item $SIG{expr}
a0d0e21e 927
14218588 928The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
929
930 sub handler { # 1st argument is signal name
fb73857a 931 my($sig) = @_;
a0d0e21e
LW
932 print "Caught a SIG$sig--shutting down\n";
933 close(LOG);
934 exit(0);
935 }
936
fb73857a
PP
937 $SIG{'INT'} = \&handler;
938 $SIG{'QUIT'} = \&handler;
a0d0e21e 939 ...
19799a22 940 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
941 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
942
f648820c
GS
943Using a value of C<'IGNORE'> usually has the effect of ignoring the
944signal, except for the C<CHLD> signal. See L<perlipc> for more about
945this special case.
946
19799a22 947Here are some other examples:
a0d0e21e 948
fb73857a 949 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 950 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 951 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
952 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
953
19799a22
GS
954Be sure not to use a bareword as the name of a signal handler,
955lest you inadvertently call it.
748a9306 956
44a8e56a
PP
957If your system has the sigaction() function then signal handlers are
958installed using it. This means you get reliable signal handling. If
959your system has the SA_RESTART flag it is used when signals handlers are
19799a22 960installed. This means that system calls for which restarting is supported
44a8e56a
PP
961continue rather than returning when a signal arrives. If you want your
962system calls to be interrupted by signal delivery then do something like
963this:
964
965 use POSIX ':signal_h';
966
967 my $alarm = 0;
968 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
969 or die "Error setting SIGALRM handler: $!\n";
970
971See L<POSIX>.
972
748a9306 973Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 974routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
975about to be printed. The warning message is passed as the first
976argument. The presence of a __WARN__ hook causes the ordinary printing
977of warnings to STDERR to be suppressed. You can use this to save warnings
978in a variable, or turn warnings into fatal errors, like this:
979
980 local $SIG{__WARN__} = sub { die $_[0] };
981 eval $proggie;
982
a8f8344d 983The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
984is about to be thrown. The error message is passed as the first
985argument. When a __DIE__ hook routine returns, the exception
986processing continues as it would have in the absence of the hook,
cb1a09d0 987unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 988The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
989can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
990
19799a22
GS
991Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
992even inside an eval(). Do not use this to rewrite a pending exception
993in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
994This strange action at a distance may be fixed in a future release
995so that C<$SIG{__DIE__}> is only called if your program is about
996to exit, as was the original intent. Any other use is deprecated.
997
998C<__DIE__>/C<__WARN__> handlers are very special in one respect:
999they may be called to report (probable) errors found by the parser.
1000In such a case the parser may be in inconsistent state, so any
1001attempt to evaluate Perl code from such a handler will probably
1002result in a segfault. This means that warnings or errors that
1003result from parsing Perl should be used with extreme caution, like
1004this:
fb73857a
PP
1005
1006 require Carp if defined $^S;
1007 Carp::confess("Something wrong") if defined &Carp::confess;
1008 die "Something wrong, but could not load Carp to give backtrace...
1009 To see backtrace try starting Perl with -MCarp switch";
1010
1011Here the first line will load Carp I<unless> it is the parser who
1012called the handler. The second line will print backtrace and die if
1013Carp was available. The third line will be executed only if Carp was
1014not available.
1015
19799a22 1016See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1017L<warnings> for additional information.
68dc0745 1018
a0d0e21e 1019=back
55602bd2
IZ
1020
1021=head2 Error Indicators
1022
19799a22
GS
1023The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1024about different types of error conditions that may appear during
1025execution of a Perl program. The variables are shown ordered by
1026the "distance" between the subsystem which reported the error and
1027the Perl process. They correspond to errors detected by the Perl
1028interpreter, C library, operating system, or an external program,
1029respectively.
55602bd2
IZ
1030
1031To illustrate the differences between these variables, consider the
19799a22 1032following Perl expression, which uses a single-quoted string:
55602bd2 1033
19799a22
GS
1034 eval q{
1035 open PIPE, "/cdrom/install |";
1036 @res = <PIPE>;
1037 close PIPE or die "bad pipe: $?, $!";
1038 };
55602bd2
IZ
1039
1040After execution of this statement all 4 variables may have been set.
1041
19799a22
GS
1042C<$@> is set if the string to be C<eval>-ed did not compile (this
1043may happen if C<open> or C<close> were imported with bad prototypes),
1044or if Perl code executed during evaluation die()d . In these cases
1045the value of $@ is the compile error, or the argument to C<die>
1046(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1047though.)
1048
1049When the eval() expression above is executed, open(), C<<PIPEE<gt>>,
1050and C<close> are translated to calls in the C run-time library and
1051thence to the operating system kernel. C<$!> is set to the C library's
1052C<errno> if one of these calls fails.
1053
1054Under a few operating systems, C<$^E> may contain a more verbose
1055error indicator, such as in this case, "CDROM tray not closed."
14218588 1056Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1057the same as C<$!>.
1058
1059Finally, C<$?> may be set to non-0 value if the external program
1060F</cdrom/install> fails. The upper eight bits reflect specific
1061error conditions encountered by the program (the program's exit()
1062value). The lower eight bits reflect mode of failure, like signal
1063death and core dump information See wait(2) for details. In
1064contrast to C<$!> and C<$^E>, which are set only if error condition
1065is detected, the variable C<$?> is set on each C<wait> or pipe
1066C<close>, overwriting the old value. This is more like C<$@>, which
1067on every eval() is always set on failure and cleared on success.
2b92dfce 1068
19799a22
GS
1069For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1070and C<$?>.
2b92dfce
GS
1071
1072=head2 Technical Note on the Syntax of Variable Names
1073
19799a22
GS
1074Variable names in Perl can have several formats. Usually, they
1075must begin with a letter or underscore, in which case they can be
1076arbitrarily long (up to an internal limit of 251 characters) and
1077may contain letters, digits, underscores, or the special sequence
1078C<::> or C<'>. In this case, the part before the last C<::> or
1079C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1080
1081Perl variable names may also be a sequence of digits or a single
1082punctuation or control character. These names are all reserved for
19799a22
GS
1083special uses by Perl; for example, the all-digits names are used
1084to hold data captured by backreferences after a regular expression
1085match. Perl has a special syntax for the single-control-character
1086names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1087character. For example, the notation C<$^W> (dollar-sign caret
1088C<W>) is the scalar variable whose name is the single character
1089control-C<W>. This is better than typing a literal control-C<W>
1090into your program.
2b92dfce 1091
87275199 1092Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1093strings that begin with control characters (or better yet, a caret).
1094These variables must be written in the form C<${^Foo}>; the braces
1095are not optional. C<${^Foo}> denotes the scalar variable whose
1096name is a control-C<F> followed by two C<o>'s. These variables are
1097reserved for future special uses by Perl, except for the ones that
1098begin with C<^_> (control-underscore or caret-underscore). No
1099control-character name that begins with C<^_> will acquire a special
1100meaning in any future version of Perl; such names may therefore be
1101used safely in programs. C<$^_> itself, however, I<is> reserved.
1102
1103Perl identifiers that begin with digits, control characters, or
2b92dfce
GS
1104punctuation characters are exempt from the effects of the C<package>
1105declaration and are always forced to be in package C<main>. A few
1106other names are also exempt:
1107
1108 ENV STDIN
1109 INC STDOUT
1110 ARGV STDERR
1111 ARGVOUT
1112 SIG
1113
1114In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1115to be in package C<main>, regardless of any C<package> declarations
2b92dfce
GS
1116presently in scope.
1117
19799a22
GS
1118=head1 BUGS
1119
1120Due to an unfortunate accident of Perl's implementation, C<use
1121English> imposes a considerable performance penalty on all regular
1122expression matches in a program, regardless of whether they occur
1123in the scope of C<use English>. For that reason, saying C<use
1124English> in libraries is strongly discouraged. See the
1125Devel::SawAmpersand module documentation from CPAN
1126(http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
1127for more information.
2b92dfce 1128
19799a22
GS
1129Having to even think about the C<$^S> variable in your exception
1130handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1131invites grievous and difficult to track down errors. Avoid it
1132and use an C<END{}> or CORE::GLOBAL::die override instead.