=head2 Predefined Names
The following names have special meaning to Perl. Most
-punctuation names have reasonable mnemonics, or analogues in one of
-the shells. Nevertheless, if you wish to use long variable names,
-you just need to say
+punctuation names have reasonable mnemonics, or analogs in the
+shells. Nevertheless, if you wish to use long variable names,
+you need only say
use English;
long names in the current package. Some even have medium names,
generally borrowed from B<awk>.
-Due to an unfortunate accident of Perl's implementation, "C<use English>"
-imposes a considerable performance penalty on all regular expression
-matches in a program, regardless of whether they occur in the scope of
-"C<use English>". For that reason, saying "C<use English>" in
-libraries is strongly discouraged. See the Devel::SawAmpersand module
-documentation from CPAN
-(http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
-for more information.
-
-To go a step further, those variables that depend on the currently
-selected filehandle may instead (and preferably) be set by calling an
-object method on the FileHandle object. (Summary lines below for this
-contain the word HANDLE.) First you must say
+If you don't mind the performance hit, variables that depend on the
+currently selected filehandle may instead be set by calling an
+appropriate object method on the IO::Handle object. (Summary lines
+below for this contain the word HANDLE.) First you must say
- use FileHandle;
+ use IO::Handle;
after which you may use either
HANDLE->method(EXPR)
-Each of the methods returns the old value of the FileHandle attribute.
+Each method returns the old value of the IO::Handle attribute.
The methods each take an optional EXPR, which if supplied specifies the
-new value for the FileHandle attribute in question. If not supplied,
-most of the methods do nothing to the current value, except for
+new value for the IO::Handle attribute in question. If not supplied,
+most methods do nothing to the current value--except for
autoflush(), which will assume a 1 for you, just to be different.
+Because loading in the IO::Handle class is an expensive operation, you should
+learn how to use the regular built-in variables.
A few of these variables are considered "read-only". This means that if
you try to assign to this variable, either directly or indirectly through
a reference, you'll raise a run-time exception.
-The following list is ordered by scalar variables first, then the
-arrays, then the hashes (except $^M was added in the wrong place).
-This is somewhat obscured by the fact that %ENV and %SIG are listed as
-$ENV{expr} and $SIG{expr}.
+You should be very careful when modifying the default values of most
+special variables described in this document. In most cases you want
+to localize these variables before changing them, since if you don't,
+the change may affect other modules which rely on the default values
+of the special variables that you have changed. This is one of the
+correct ways to read the whole file at once:
+
+ open my $fh, "foo" or die $!;
+ local $/; # enable localized slurp mode
+ my $content = <$fh>;
+ close $fh;
+
+But the following code is quite bad:
+
+ open my $fh, "foo" or die $!;
+ undef $/; # enable slurp mode
+ my $content = <$fh>;
+ close $fh;
+
+since some other module, may want to read data from some file in the
+default "line mode", so if the code we have just presented has been
+executed, the global value of C<$/> is now changed for any other code
+running inside the same Perl interpreter.
+
+Usually when a variable is localized you want to make sure that this
+change affects the shortest scope possible. So unless you are already
+inside some short C<{}> block, you should create one yourself. For
+example:
+
+ my $content = '';
+ open my $fh, "foo" or die $!;
+ {
+ local $/;
+ $content = <$fh>;
+ }
+ close $fh;
+
+Here is an example of how your own code can go broken:
+
+ for (1..5){
+ nasty_break();
+ print "$_ ";
+ }
+ sub nasty_break {
+ $_ = 5;
+ # do something with $_
+ }
+
+You probably expect this code to print:
+
+ 1 2 3 4 5
+
+but instead you get:
+
+ 5 5 5 5 5
+
+Why? Because nasty_break() modifies C<$_> without localizing it
+first. The fix is to add local():
+
+ local $_ = 5;
+It's easy to notice the problem in such a short example, but in more
+complicated code you are looking for trouble if you don't localize
+changes to the special variables.
+
+The following list is ordered by scalar variables first, then the
+arrays, then the hashes.
=over 8
The default input and pattern-searching space. The following pairs are
equivalent:
- while (<>) {...} # equivalent in only while!
+ while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
- chop
- chop($_)
+ chomp
+ chomp($_)
Here are the places where Perl will assume $_ even if you
don't use it:
=item *
-The default place to put an input record when a C<E<lt>FHE<gt>>
+The default place to put an input record when a C<< <FH> >>
operation's result is tested by itself as the sole criterion of a C<while>
-test. Note that outside of a C<while> test, this will not happen.
+test. Outside a C<while> test, this will not happen.
=back
=over 8
-=item $E<lt>I<digits>E<gt>
+=item $a
+
+=item $b
+
+Special package variables when using sort(), see L<perlfunc/sort>.
+Because of this specialness $a and $b don't need to be declared
+(using local(), use vars, or our()) even when using the strict
+vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
+if you want to be able to use them in the sort() comparison block
+or function.
+
+=back
+
+=over 8
+
+=item $<I<digits>>
-Contains the subpattern from the corresponding set of parentheses in
-the last pattern matched, not counting patterns matched in nested
-blocks that have been exited already. (Mnemonic: like \digits.)
-These variables are all read-only.
+Contains the subpattern from the corresponding set of capturing
+parentheses from the last pattern match, not counting patterns
+matched in nested blocks that have been exited already. (Mnemonic:
+like \digits.) These variables are all read-only and dynamically
+scoped to the current BLOCK.
=item $MATCH
The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or eval() enclosed by the current
-BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
+BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
+and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See the
-Devel::SawAmpersand module from CPAN for more information.
+performance penalty on all regular expression matches. See L</BUGS>.
=item $PREMATCH
string.) This variable is read-only.
The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See the
-Devel::SawAmpersand module from CPAN for more information.
+performance penalty on all regular expression matches. See L</BUGS>.
=item $POSTMATCH
enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
string.) Example:
- $_ = 'abcdefghi';
+ local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
-This variable is read-only.
+This variable is read-only and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See the
-Devel::SawAmpersand module from CPAN for more information.
+performance penalty on all regular expression matches. See L</BUGS>.
=item $LAST_PAREN_MATCH
=item $+
-The last bracket matched by the last search pattern. This is useful if
-you don't know which of a set of alternative patterns matched. For
-example:
+The text matched by the last bracket of the last successful search pattern.
+This is useful if you don't know which one of a set of alternative patterns
+matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
-This variable is read-only.
+This variable is read-only and dynamically scoped to the current BLOCK.
-=item @+
+=item $^N
-$+[0] is the offset of the end of the last successfull match.
-C<$+[>I<n>C<]> is the offset of the end of the substring matched by
-I<n>-th subpattern, or undef if the subpattern did not match.
+The text matched by the used group most-recently closed (i.e. the group
+with the rightmost closing parenthesis) of the last successful search
+pattern. (Mnemonic: the (possibly) Nested parenthesis that most
+recently closed.)
-Thus after a match against $_, $& coincides with C<substr $_, $-[0],
-$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
-$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
-C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number
-of subgroups in the last successful match. Note the difference with
-C<$#->, which is the last I<matched> subgroup. Compare with L<"@-">.
+This is primarily used inside C<(?{...})> blocks for examining text
+recently matched. For example, to effectively capture text to a variable
+(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
+
+ (?:(...)(?{ $var = $^N }))
+
+By setting and then using C<$var> in this way relieves you from having to
+worry about exactly which numbered set of parentheses they are.
+
+This variable is dynamically scoped to the current BLOCK.
+
+=item @LAST_MATCH_END
+
+=item @+
+
+This array holds the offsets of the ends of the last successful
+submatches in the currently active dynamic scope. C<$+[0]> is
+the offset into the string of the end of the entire match. This
+is the same value as what the C<pos> function returns when called
+on the variable that was matched against. The I<n>th element
+of this array holds the offset of the I<n>th submatch, so
+C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
+past where $2 ends, and so on. You can use C<$#+> to determine
+how many subgroups were in the last successful match. See the
+examples given for the C<@-> variable.
=item $MULTILINE_MATCHING
=item $*
-Set to 1 to do multi-line matching within a string, 0 to tell Perl
-that it can assume that strings contain a single line, for the purpose
-of optimizing pattern matches. Pattern matches on strings containing
-multiple newlines can produce confusing results when "C<$*>" is 0. Default
-is 0. (Mnemonic: * matches multiple things.) Note that this variable
-influences the interpretation of only "C<^>" and "C<$>". A literal newline can
-be searched for even when C<$* == 0>.
+Set to a non-zero integer value to do multi-line matching within a
+string, 0 (or undefined) to tell Perl that it can assume that strings
+contain a single line, for the purpose of optimizing pattern matches.
+Pattern matches on strings containing multiple newlines can produce
+confusing results when C<$*> is 0 or undefined. Default is undefined.
+(Mnemonic: * matches multiple things.) This variable influences the
+interpretation of only C<^> and C<$>. A literal newline can be searched
+for even when C<$* == 0>.
-Use of "C<$*>" is deprecated in modern Perls, supplanted by
+Use of C<$*> is deprecated in modern Perl, supplanted by
the C</s> and C</m> modifiers on pattern matching.
-=item input_line_number HANDLE EXPR
+Assigning a non-numerical value to C<$*> triggers a warning (and makes
+C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
+makes that an implicit C<int> is applied on the value.
+
+=item HANDLE->input_line_number(EXPR)
=item $INPUT_LINE_NUMBER
=item $.
-The current input line number for the last file handle from
-which you read (or performed a C<seek> or C<tell> on). An
-explicit close on a filehandle resets the line number. Because
-"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
-across ARGV files (but see examples under eof()). Localizing C<$.> has
-the effect of also localizing Perl's notion of "the last read
-filehandle". (Mnemonic: many programs use "." to mean the current line
-number.)
+Current line number for the last filehandle accessed.
+
+Each filehandle in Perl counts the number of lines that have been read
+from it. (Depending on the value of C<$/>, Perl's idea of what
+constitutes a line may not match yours.) When a line is read from a
+filehandle (via readline() or C<< <> >>), or when tell() or seek() is
+called on it, C<$.> becomes an alias to the line counter for that
+filehandle.
-=item input_record_separator HANDLE EXPR
+You can adjust the counter by assigning to C<$.>, but this will not
+actually move the seek pointer. I<Localizing C<$.> will not localize
+the filehandle's line count>. Instead, it will localize perl's notion
+of which filehandle C<$.> is currently aliased to.
+
+C<$.> is reset when the filehandle is closed, but B<not> when an open
+filehandle is reopened without an intervening close(). For more
+details, see L<perlop/"I/O Operators">. Because C<< <> >> never does
+an explicit close, line numbers increase across ARGV files (but see
+examples in L<perlfunc/eof>).
+
+You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
+line counter for a given filehandle without having to worry about
+which handle you last accessed.
+
+(Mnemonic: many programs use "." to mean the current line number.)
+
+=item IO::Handle->input_record_separator(EXPR)
=item $INPUT_RECORD_SEPARATOR
=item $/
-The input record separator, newline by default. Works like B<awk>'s RS
-variable, including treating empty lines as delimiters if set to the
-null string. (Note: An empty line cannot contain any spaces or tabs.)
-You may set it to a multi-character string to match a multi-character
-delimiter, or to C<undef> to read to end of file. Note that setting it
-to C<"\n\n"> means something slightly different than setting it to
-C<"">, if the file contains consecutive empty lines. Setting it to
-C<""> will treat two or more consecutive empty lines as a single empty
-line. Setting it to C<"\n\n"> will blindly assume that the next input
-character belongs to the next paragraph, even if it's a newline.
-(Mnemonic: / is used to delimit line boundaries when quoting poetry.)
-
- undef $/;
- $_ = <FH>; # whole file now here
+The input record separator, newline by default. This
+influences Perl's idea of what a "line" is. Works like B<awk>'s RS
+variable, including treating empty lines as a terminator if set to
+the null string. (An empty line cannot contain any spaces
+or tabs.) You may set it to a multi-character string to match a
+multi-character terminator, or to C<undef> to read through the end
+of file. Setting it to C<"\n\n"> means something slightly
+different than setting to C<"">, if the file contains consecutive
+empty lines. Setting to C<""> will treat two or more consecutive
+empty lines as a single empty line. Setting to C<"\n\n"> will
+blindly assume that the next input character belongs to the next
+paragraph, even if it's a newline. (Mnemonic: / delimits
+line boundaries when quoting poetry.)
+
+ local $/; # enable "slurp" mode
+ local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
-Remember: the value of $/ is a string, not a regexp. AWK has to be
-better for something :-)
+Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
+better for something. :-)
-Setting $/ to a reference to an integer, scalar containing an integer, or
-scalar that's convertable to an integer will attempt to read records
+Setting C<$/> to a reference to an integer, scalar containing an integer, or
+scalar that's convertible to an integer will attempt to read records
instead of lines, with the maximum record size being the referenced
-integer. So this:
+integer. So this:
- $/ = \32768; # or \"32768", or \$var_containing_32768
- open(FILE, $myfile);
- $_ = <FILE>;
+ local $/ = \32768; # or \"32768", or \$var_containing_32768
+ open my $fh, $myfile or die $!;
+ local $_ = <$fh>;
-will read a record of no more than 32768 bytes from FILE. If you're not
-reading from a record-oriented file (or your OS doesn't have
-record-oriented files), then you'll likely get a full chunk of data with
-every read. If a record is larger than the record size you've set, you'll
-get the record back in pieces.
+will read a record of no more than 32768 bytes from FILE. If you're
+not reading from a record-oriented file (or your OS doesn't have
+record-oriented files), then you'll likely get a full chunk of data
+with every read. If a record is larger than the record size you've
+set, you'll get the record back in pieces.
-On VMS, record reads are done with the equivalent of C<sysread>, so it's
-best not to mix record and non-record reads on the same file. (This is
-likely not a problem, as any file you'd want to read in record mode is
-proably usable in line mode) Non-VMS systems perform normal I/O, so
-it's safe to mix record and non-record reads of a file.
+On VMS, record reads are done with the equivalent of C<sysread>,
+so it's best not to mix record and non-record reads on the same
+file. (This is unlikely to be a problem, because any file you'd
+want to read in record mode is probably unusable in line mode.)
+Non-VMS systems do normal I/O, so it's safe to mix record and
+non-record reads of a file.
-=item autoflush HANDLE EXPR
+See also L<perlport/"Newlines">. Also see C<$.>.
+
+=item HANDLE->autoflush(EXPR)
=item $OUTPUT_AUTOFLUSH
=item $|
-If set to nonzero, forces a flush right away and after every write or print on the
-currently selected output channel. Default is 0 (regardless of whether
-the channel is actually buffered by the system or not; C<$|> tells you
-only whether you've asked Perl explicitly to flush after each write).
-Note that STDOUT will typically be line buffered if output is to the
-terminal and block buffered otherwise. Setting this variable is useful
-primarily when you are outputting to a pipe, such as when you are running
-a Perl script under rsh and want to see the output as it's happening. This
-has no effect on input buffering.
-(Mnemonic: when you want your pipes to be piping hot.)
+If set to nonzero, forces a flush right away and after every write
+or print on the currently selected output channel. Default is 0
+(regardless of whether the channel is really buffered by the
+system or not; C<$|> tells you only whether you've asked Perl
+explicitly to flush after each write). STDOUT will
+typically be line buffered if output is to the terminal and block
+buffered otherwise. Setting this variable is useful primarily when
+you are outputting to a pipe or socket, such as when you are running
+a Perl program under B<rsh> and want to see the output as it's
+happening. This has no effect on input buffering. See L<perlfunc/getc>
+for that. (Mnemonic: when you want your pipes to be piping hot.)
-=item output_field_separator HANDLE EXPR
+=item IO::Handle->output_field_separator EXPR
=item $OUTPUT_FIELD_SEPARATOR
=item $,
The output field separator for the print operator. Ordinarily the
-print operator simply prints out the comma-separated fields you
-specify. To get behavior more like B<awk>, set this variable
-
as you would set B<awk>'s OFS variable to specify what is printed
-between fields. (Mnemonic: what is printed when there is a , in your
-print statement.)
+print operator simply prints out its arguments without further
+adornment. To get behavior more like B<awk>, set this variable as
+you would set B<awk>'s OFS variable to specify what is printed
+between fields. (Mnemonic: what is printed when there is a "," in
+your print statement.)
-=item output_record_separator HANDLE EXPR
+=item IO::Handle->output_record_separator EXPR
=item $OUTPUT_RECORD_SEPARATOR
=item $\
The output record separator for the print operator. Ordinarily the
-print operator simply prints out the comma-separated fields you
-specify, with no trailing newline or record separator assumed.
-To get behavior more like B<awk>, set this variable as you would
-
set B<awk>'s ORS variable to specify what is printed at the end of the
-print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
-print. Also, it's just like C<$/>, but it's what you get "back" from
-Perl.)
+print operator simply prints out its arguments as is, with no
+trailing newline or other end-of-record string added. To get
+behavior more like B<awk>, set this variable as you would set
+B<awk>'s ORS variable to specify what is printed at the end of the
+print. (Mnemonic: you set C<$\> instead of adding "\n" at the
+end of the print. Also, it's just like C<$/>, but it's what you
+get "back" from Perl.)
=item $LIST_SEPARATOR
=item $"
-This is like "C<$,>" except that it applies to array values interpolated
-into a double-quoted string (or similar interpreted string). Default
-is a space. (Mnemonic: obvious, I think.)
+This is like C<$,> except that it applies to array and slice values
+interpolated into a double-quoted string (or similar interpreted
+string). Default is a space. (Mnemonic: obvious, I think.)
=item $SUBSCRIPT_SEPARATOR
($foo{$a},$foo{$b},$foo{$c})
-Default is "\034", the same as SUBSEP in B<awk>. Note that if your
-keys contain binary data there might not be any safe value for "C<$;>".
+Default is "\034", the same as SUBSEP in B<awk>. If your
+keys contain binary data there might not be any safe value for C<$;>.
(Mnemonic: comma (the syntactic subscript separator) is a
-semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
+semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
taken for something more important.)
-Consider using "real" multidimensional arrays.
+Consider using "real" multidimensional arrays as described
+in L<perllol>.
=item $OFMT
The output format for printed numbers. This variable is a half-hearted
attempt to emulate B<awk>'s OFMT variable. There are times, however,
-when B<awk> and Perl have differing notions of what is in fact
-numeric. The initial value is %.I<n>g, where I<n> is the value
+when B<awk> and Perl have differing notions of what counts as
+numeric. The initial value is "%.I<n>g", where I<n> is the value
of the macro DBL_DIG from your system's F<float.h>. This is different from
-B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
+B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
-Use of "C<$#>" is deprecated.
+Use of C<$#> is deprecated.
-=item format_page_number HANDLE EXPR
+=item HANDLE->format_page_number(EXPR)
=item $FORMAT_PAGE_NUMBER
=item $%
The current page number of the currently selected output channel.
+Used with formats.
(Mnemonic: % is page number in B<nroff>.)
-=item format_lines_per_page HANDLE EXPR
+=item HANDLE->format_lines_per_page(EXPR)
=item $FORMAT_LINES_PER_PAGE
=item $=
The current page length (printable lines) of the currently selected
-output channel. Default is 60. (Mnemonic: = has horizontal lines.)
+output channel. Default is 60.
+Used with formats.
+(Mnemonic: = has horizontal lines.)
-=item format_lines_left HANDLE EXPR
+=item HANDLE->format_lines_left(EXPR)
=item $FORMAT_LINES_LEFT
=item $-
The number of lines left on the page of the currently selected output
-channel. (Mnemonic: lines_on_page - lines_printed.)
+channel.
+Used with formats.
+(Mnemonic: lines_on_page - lines_printed.)
+
+=item @LAST_MATCH_START
=item @-
-$-[0] is the offset of the start of the last successfull match.
+$-[0] is the offset of the start of the last successful match.
C<$-[>I<n>C<]> is the offset of the start of the substring matched by
I<n>-th subpattern, or undef if the subpattern did not match.
$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
-matched subgroup in the last successful match. Note the difference with
-C<$#+>, which is the number of subgroups in the regular expression. Compare
-with L<"@+">.
+matched subgroup in the last successful match. Contrast with
+C<$#+>, the number of subgroups in the regular expression. Compare
+with C<@+>.
+
+This array holds the offsets of the beginnings of the last
+successful submatches in the currently active dynamic scope.
+C<$-[0]> is the offset into the string of the beginning of the
+entire match. The I<n>th element of this array holds the offset
+of the I<n>th submatch, so C<$-[1]> is the offset where $1
+begins, C<$-[2]> the offset where $2 begins, and so on.
+
+After a match against some variable $var:
-=item format_name HANDLE EXPR
+=over 5
+
+=item C<$`> is the same as C<substr($var, 0, $-[0])>
+
+=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
+
+=item C<$'> is the same as C<substr($var, $+[0])>
+
+=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
+
+=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
+
+=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
+
+=back
+
+=item HANDLE->format_name(EXPR)
=item $FORMAT_NAME
=item $~
The name of the current report format for the currently selected output
-channel. Default is name of the filehandle. (Mnemonic: brother to
-"C<$^>".)
+channel. Default is the name of the filehandle. (Mnemonic: brother to
+C<$^>.)
-=item format_top_name HANDLE EXPR
+=item HANDLE->format_top_name(EXPR)
=item $FORMAT_TOP_NAME
=item $^
The name of the current top-of-page format for the currently selected
-output channel. Default is name of the filehandle with _TOP
+output channel. Default is the name of the filehandle with _TOP
appended. (Mnemonic: points to top of page.)
-=item format_line_break_characters HANDLE EXPR
+=item IO::Handle->format_line_break_characters EXPR
=item $FORMAT_LINE_BREAK_CHARACTERS
S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
poetry is a part of a line.)
-=item format_formfeed HANDLE EXPR
+=item IO::Handle->format_formfeed EXPR
=item $FORMAT_FORMFEED
=item $^L
-What formats output to perform a form feed. Default is \f.
+What formats output as a form feed. Default is \f.
=item $ACCUMULATOR
=item $^A
The current value of the write() accumulator for format() lines. A format
-contains formline() commands that put their result into C<$^A>. After
+contains formline() calls that put their result into C<$^A>. After
calling its format, write() prints out the contents of C<$^A> and empties.
-So you never actually see the contents of C<$^A> unless you call
+So you never really see the contents of C<$^A> unless you call
formline() yourself and then look at it. See L<perlform> and
L<perlfunc/formline()>.
=item $?
The status returned by the last pipe close, backtick (C<``>) command,
-or system() operator. Note that this is the status word returned by the
-wait() system call (or else is made up to look like it). Thus, the exit
-value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and C<$? & 127>
-gives which signal, if any, the process died from, and C<$? & 128> reports
-whether there was a core dump. (Mnemonic: similar to B<sh> and B<ksh>.)
+successful call to wait() or waitpid(), or from the system()
+operator. This is just the 16-bit status word returned by the
+wait() system call (or else is made up to look like it). Thus, the
+exit value of the subprocess is really (C<<< $? >> 8 >>>), and
+C<$? & 127> gives which signal, if any, the process died from, and
+C<$? & 128> reports whether there was a core dump. (Mnemonic:
+similar to B<sh> and B<ksh>.)
Additionally, if the C<h_errno> variable is supported in C, its value
-is returned via $? if any of the C<gethost*()> functions fail.
+is returned via $? if any C<gethost*()> function fails.
-Note that if you have installed a signal handler for C<SIGCHLD>, the
+If you have installed a signal handler for C<SIGCHLD>, the
value of C<$?> will usually be wrong outside that handler.
Inside an C<END> subroutine C<$?> contains the value that is going to be
given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
-change the exit status of the script.
+change the exit status of your program. For example:
+
+ END {
+ $? = 1 if $? == 255; # die would make it 255
+ }
Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
actual VMS exit status, instead of the default emulation of POSIX
-status.
+status; see L<perlvms/$?> for details.
Also see L<Error Indicators>.
+=item ${^ENCODING}
+
+The encoding used to interpret native eight-bit encodings to Unicode,
+see L<encode>. An opaque C<Encode::XS> object.
+
=item $OS_ERROR
=item $ERRNO
=item $!
-If used in a numeric context, yields the current value of errno, with
-all the usual caveats. (This means that you shouldn't depend on the
-value of C<$!> to be anything in particular unless you've gotten a
-specific error return indicating a system error.) If used in a string
-context, yields the corresponding system error string. You can assign
-to C<$!> to set I<errno> if, for instance, you want C<"$!"> to return the
-string for error I<n>, or you want to set the exit value for the die()
-operator. (Mnemonic: What just went bang?)
+If used numerically, yields the current value of the C C<errno>
+variable, with all the usual caveats. (This means that you shouldn't
+depend on the value of C<$!> to be anything in particular unless
+you've gotten a specific error return indicating a system error.)
+If used an a string, yields the corresponding system error string.
+You can assign a number to C<$!> to set I<errno> if, for instance,
+you want C<"$!"> to return the string for error I<n>, or you want
+to set the exit value for the die() operator. (Mnemonic: What just
+went bang?)
Also see L<Error Indicators>.
Under Win32, C<$^E> always returns the last error information
reported by the Win32 call C<GetLastError()> which describes
the last error from within the Win32 API. Most Win32-specific
-code will report errors via C<$^E>. ANSI C and UNIX-like calls
+code will report errors via C<$^E>. ANSI C and Unix-like calls
set C<errno> and so most portable Perl code will report errors
via C<$!>.
=item $@
-The Perl syntax error message from the last eval() command. If null, the
-last eval() parsed and executed correctly (although the operations you
-invoked may have failed in the normal fashion). (Mnemonic: Where was
-the syntax error "at"?)
+The Perl syntax error message from the last eval() operator.
+If $@ is the null string, the last eval() parsed and executed
+correctly (although the operations you invoked may have failed in the
+normal fashion). (Mnemonic: Where was the syntax error "at"?)
-Note that warning messages are not collected in this variable. You can,
+Warning messages are not collected in this variable. You can,
however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
as described below.
=item $$
-The process number of the Perl running this script. (Mnemonic: same
-as shells.)
+The process number of the Perl running this script. You should
+consider this variable read-only, although it will be altered
+across fork() calls. (Mnemonic: same as shells.)
=item $REAL_USER_ID
=item $<
-The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
-if you're running setuid.)
+The real uid of this process. (Mnemonic: it's the uid you came I<from>,
+if you're running setuid.) You can change both the real uid and
+the effective uid at the same time by using POSIX::setuid().
=item $EFFECTIVE_USER_ID
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
-(Mnemonic: it's the uid you went I<TO>, if you're running setuid.)
-Note: "C<$E<lt>>" and "C<$E<gt>>" can be swapped only on machines
+You can change both the effective uid and the real uid at the same
+time by using POSIX::setuid().
+
+(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
+C<< $< >> and C<< $> >> can be swapped only on machines
supporting setreuid().
=item $REAL_GROUP_ID
getgid(), and the subsequent ones by getgroups(), one of which may be
the same as the first number.
-However, a value assigned to "C<$(>" must be a single number used to
-set the real gid. So the value given by "C<$(>" should I<not> be assigned
-back to "C<$(>" without being forced numeric, such as by adding zero.
+However, a value assigned to C<$(> must be a single number used to
+set the real gid. So the value given by C<$(> should I<not> be assigned
+back to C<$(> without being forced numeric, such as by adding zero.
+
+You can change both the real gid and the effective gid at the same
+time by using POSIX::setgid().
-(Mnemonic: parentheses are used to I<GROUP> things. The real gid is the
-group you I<LEFT>, if you're running setgid.)
+(Mnemonic: parentheses are used to I<group> things. The real gid is the
+group you I<left>, if you're running setgid.)
=item $EFFECTIVE_GROUP_ID
returned by getegid(), and the subsequent ones by getgroups(), one of
which may be the same as the first number.
-Similarly, a value assigned to "C<$)>" must also be a space-separated
-list of numbers. The first number is used to set the effective gid, and
+Similarly, a value assigned to C<$)> must also be a space-separated
+list of numbers. The first number sets the effective gid, and
the rest (if any) are passed to setgroups(). To get the effect of an
empty list for setgroups(), just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty setgroups()
list, say C< $) = "5 5" >.
-(Mnemonic: parentheses are used to I<GROUP> things. The effective gid
-is the group that's I<RIGHT> for you, if you're running setgid.)
+You can change both the effective gid and the real gid at the same
+time by using POSIX::setgid() (use only a single numeric argument).
+
+(Mnemonic: parentheses are used to I<group> things. The effective gid
+is the group that's I<right> for you, if you're running setgid.)
-Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
-machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
-and "C<$)>" can be swapped only on machines supporting setregid().
+C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
+machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
+and C<$)> can be swapped only on machines supporting setregid().
=item $PROGRAM_NAME
=item $0
-Contains the name of the file containing the Perl script being
-executed. On some operating systems
-assigning to "C<$0>" modifies the argument area that the ps(1)
-program sees. This is more useful as a way of indicating the
-current program state than it is for hiding the program you're running.
+Contains the name of the program being executed. On some operating
+systems assigning to C<$0> modifies the argument area that the B<ps>
+program sees. This is more useful as a way of indicating the current
+program state than it is for hiding the program you're running.
(Mnemonic: same as B<sh> and B<ksh>.)
+Note for BSD users: setting C<$0> does not completely remove "perl"
+from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
+result in C<"perl: foobar (perl)">. This is an operating system
+feature.
+
=item $[
The index of the first element in an array, and of the first character
-in a substring. Default is 0, but you could set it to 1 to make
-Perl behave more like B<awk> (or Fortran) when subscripting and when
-evaluating the index() and substr() functions. (Mnemonic: [ begins
-subscripts.)
-
-As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
-and cannot influence the behavior of any other file. Its use is
-discouraged.
+in a substring. Default is 0, but you could theoretically set it
+to 1 to make Perl behave more like B<awk> (or Fortran) when
+subscripting and when evaluating the index() and substr() functions.
+(Mnemonic: [ begins subscripts.)
-=item $PERL_VERSION
+As of release 5 of Perl, assignment to C<$[> is treated as a compiler
+directive, and cannot influence the behavior of any other file.
+Its use is highly discouraged.
=item $]
warn "No checksumming!\n" if $] < 3.019;
See also the documentation of C<use VERSION> and C<require VERSION>
-for a convenient way to fail if the Perl interpreter is too old.
+for a convenient way to fail if the running Perl interpreter is too old.
+
+The floating point representation can sometimes lead to inaccurate
+numeric comparisons. See C<$^V> for a more modern representation of
+the Perl version that allows accurate string comparisons.
=item $COMPILING
=item $^C
-The current value of the flag associated with the B<-c> switch. Mainly
-of use with B<-MO=...> to allow code to alter its behaviour when being compiled.
-(For example to automatically AUTOLOADing at compile time rather than normal
-deferred loading.) Setting C<$^C = 1> is similar to calling C<B::minus_c>.
+The current value of the flag associated with the B<-c> switch.
+Mainly of use with B<-MO=...> to allow code to alter its behavior
+when being compiled, such as for example to AUTOLOAD at compile
+time rather than normal, deferred loading. See L<perlcc>. Setting
+C<$^C = 1> is similar to calling C<B::minus_c>.
=item $DEBUGGING
descriptors are passed to exec()ed processes, while higher file
descriptors are not. Also, during an open(), system file descriptors are
preserved even if the open() fails. (Ordinary file descriptors are
-closed before the open() is attempted.) Note that the close-on-exec
+closed before the open() is attempted.) The close-on-exec
status of a file descriptor will be decided according to the value of
-C<$^F> when the open() or pipe() was called, not the time of the exec().
+C<$^F> when the corresponding file, pipe, or socket was opened, not the
+time of the exec().
=item $^H
-The current set of syntax checks enabled by C<use strict> and other block
-scoped compiler hints. See the documentation of C<strict> for more details.
+WARNING: This variable is strictly for internal use only. Its availability,
+behavior, and contents are subject to change without notice.
+
+This variable contains compile-time hints for the Perl interpreter. At the
+end of compilation of a BLOCK the value of this variable is restored to the
+value when the interpreter started to compile the BLOCK.
+
+When perl begins to parse any block construct that provides a lexical scope
+(e.g., eval body, required file, subroutine body, loop body, or conditional
+block), the existing value of $^H is saved, but its value is left unchanged.
+When the compilation of the block is completed, it regains the saved value.
+Between the points where its value is saved and restored, code that
+executes within BEGIN blocks is free to change the value of $^H.
+
+This behavior provides the semantic of lexical scoping, and is used in,
+for instance, the C<use strict> pragma.
+
+The contents should be an integer; different bits of it are used for
+different pragmatic flags. Here's an example:
+
+ sub add_100 { $^H |= 0x100 }
+
+ sub foo {
+ BEGIN { add_100() }
+ bar->baz($boon);
+ }
+
+Consider what happens during execution of the BEGIN block. At this point
+the BEGIN block has already been compiled, but the body of foo() is still
+being compiled. The new value of $^H will therefore be visible only while
+the body of foo() is being compiled.
+
+Substitution of the above BEGIN block with:
+
+ BEGIN { require strict; strict->import('vars') }
+
+demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
+version of the same lexical pragma:
+
+ BEGIN { require strict; strict->import('vars') if $condition }
+
+=item %^H
+
+WARNING: This variable is strictly for internal use only. Its availability,
+behavior, and contents are subject to change without notice.
+
+The %^H hash provides the same scoping semantic as $^H. This makes it
+useful for implementation of lexically scoped pragmas.
=item $INPLACE_EDIT
=item $^M
-By default, running out of memory it is not trappable. However, if
-compiled for this, Perl may use the contents of C<$^M> as an emergency
-pool after die()ing with this message. Suppose that your Perl were
-compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc. Then
+By default, running out of memory is an untrappable, fatal error.
+However, if suitably built, Perl can use the contents of C<$^M>
+as an emergency memory pool after die()ing. Suppose that your Perl
+were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
+Then
- $^M = 'a' x (1<<16);
+ $^M = 'a' x (1 << 16);
-would allocate a 64K buffer for use when in emergency. See the F<INSTALL>
-file for information on how to enable this option. As a disincentive to
-casual use of this advanced feature, there is no L<English> long name for
-this variable.
+would allocate a 64K buffer for use in an emergency. See the
+F<INSTALL> file in the Perl distribution for information on how to
+enable this option. To discourage casual use of this advanced
+feature, there is no L<English|English> long name for this variable.
=item $OSNAME
The name of the operating system under which this copy of Perl was
built, as determined during the configuration process. The value
-is identical to C<$Config{'osname'}>.
+is identical to C<$Config{'osname'}>. See also L<Config> and the
+B<-V> command-line switch documented in L<perlrun>.
+
+=item ${^OPEN}
+
+An internal variable used by PerlIO. A string in two parts, separated
+by a C<\0> byte, the first part is the input disciplines, the second
+part is the output disciplines.
=item $PERLDB
=item $^P
-The internal variable for debugging support. Different bits mean the
-following (subject to change):
+The internal variable for debugging support. The meanings of the
+various bits are subject to change, but currently indicate:
=over 6
Start with single-step on.
+=item 0x40
+
+Use subroutine address instead of name when reporting.
+
+=item 0x80
+
+Report C<goto &subroutine> as well.
+
+=item 0x100
+
+Provide informative "file" names for evals based on the place they were compiled.
+
+=item 0x200
+
+Provide informative names to anonymous subroutines based on the place they
+were compiled.
+
=back
-Note that some bits may be relevent at compile-time only, some at
-run-time only. This is a new mechanism and the details may change.
+Some bits may be relevant at compile-time only, some at
+run-time only. This is a new mechanism and the details may change.
+
+=item $LAST_REGEXP_CODE_RESULT
=item $^R
-The result of evaluation of the last successful L<perlre/C<(?{ code })>>
-regular expression assertion. (Excluding those used as switches.) May
-be written to.
+The result of evaluation of the last successful C<(?{ code })>
+regular expression assertion (see L<perlre>). May be written to.
+
+=item $EXCEPTIONS_BEING_CAUGHT
=item $^S
Current state of the interpreter. Undefined if parsing of the current
module/eval is not finished (may happen in $SIG{__DIE__} and
-$SIG{__WARN__} handlers). True if inside an eval, otherwise false.
+$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
=item $BASETIME
=item $^T
-The time at which the script began running, in seconds since the
+The time at which the program began running, in seconds since the
epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
-and B<-C> filetests are
-based on this value.
+and B<-C> filetests are based on this value.
+
+=item ${^TAINT}
+
+Reflects if taint mode is on or off (i.e. if the program was run with
+B<-T> or not). True for on, false for off.
+
+=item $PERL_VERSION
+
+=item $^V
+
+The revision, version, and subversion of the Perl interpreter, represented
+as a string composed of characters with those ordinals. Thus in Perl v5.6.0
+it equals C<chr(5) . chr(6) . chr(0)> and will return true for
+C<$^V eq v5.6.0>. Note that the characters in this string value can
+potentially be in Unicode range.
+
+This can be used to determine whether the Perl interpreter executing a
+script is in the right range of versions. (Mnemonic: use ^V for Version
+Control.) Example:
+
+ warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
+
+See the documentation of C<use VERSION> and C<require VERSION>
+for a convenient way to fail if the running Perl interpreter is too old.
+
+See also C<$]> for an older representation of the Perl version.
=item $WARNING
=item $^W
-The current value of the warning switch, either TRUE or FALSE.
-(Mnemonic: related to the B<-w> switch.)
+The current value of the warning switch, initially true if B<-w>
+was used, false otherwise, but directly modifiable. (Mnemonic:
+related to the B<-w> switch.) See also L<warnings>.
+
+=item ${^WARNING_BITS}
+
+The current set of warning checks enabled by the C<use warnings> pragma.
+See the documentation of C<warnings> for more details.
+
+=item ${^WIDE_SYSTEM_CALLS}
+
+Global flag that enables system calls made by Perl to use wide character
+APIs native to the system, if available. This is currently only implemented
+on the Windows platform.
+
+This can also be enabled from the command line using the C<-C> switch.
+
+The initial value is typically C<0> for compatibility with Perl versions
+earlier than 5.6, but may be automatically set to C<1> by Perl if the system
+provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
+
+The C<bytes> pragma always overrides the effect of this flag in the current
+lexical scope. See L<bytes>.
=item $EXECUTABLE_NAME
=item $^X
The name that the Perl binary itself was executed as, from C's C<argv[0]>.
+This may not be a full pathname, nor even necessarily in your path.
+
+=item ARGV
+
+The special filehandle that iterates over command-line filenames in
+C<@ARGV>. Usually written as the null filehandle in the angle operator
+C<< <> >>. Note that currently C<ARGV> only has its magical effect
+within the C<< <> >> operator; elsewhere it is just a plain filehandle
+corresponding to the last file opened by C<< <> >>. In particular,
+passing C<\*ARGV> as a parameter to a function that expects a filehandle
+may not cause your function to automatically read the contents of all the
+files in C<@ARGV>.
=item $ARGV
-contains the name of the current file when reading from E<lt>E<gt>.
+contains the name of the current file when reading from <>.
=item @ARGV
-The array @ARGV contains the command line arguments intended for the
-script. Note that C<$#ARGV> is the generally number of arguments minus
-one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
-"C<$0>" for the command name.
+The array @ARGV contains the command-line arguments intended for
+the script. C<$#ARGV> is generally the number of arguments minus
+one, because C<$ARGV[0]> is the first argument, I<not> the program's
+command name itself. See C<$0> for the command name.
+
+=item @F
+
+The array @F contains the fields of each line read in when autosplit
+mode is turned on. See L<perlrun> for the B<-a> switch. This array
+is package-specific, and must be declared or given a full package name
+if not in package main when running under C<strict 'vars'>.
=item @INC
-The array @INC contains the list of places to look for Perl scripts to
-be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
-initially consists of the arguments to any B<-I> command line switches,
-followed by the default Perl library, probably F</usr/local/lib/perl>,
-followed by ".", to represent the current directory. If you need to
-modify this at runtime, you should use the C<use lib> pragma
-to get the machine-dependent library properly loaded also:
+The array @INC contains the list of places that the C<do EXPR>,
+C<require>, or C<use> constructs look for their library files. It
+initially consists of the arguments to any B<-I> command-line
+switches, followed by the default Perl library, probably
+F</usr/local/lib/perl>, followed by ".", to represent the current
+directory. If you need to modify this at runtime, you should use
+the C<use lib> pragma to get the machine-dependent library properly
+loaded also:
use lib '/mypath/libdir/';
use SomeMod;
+You can also insert hooks into the file inclusion system by putting Perl
+code directly into @INC. Those hooks may be subroutine references, array
+references or blessed objects. See L<perlfunc/require> for details.
+
=item @_
Within a subroutine the array @_ contains the parameters passed to that
-subroutine. See L<perlsub>.
+subroutine.
See L<perlsub>.
=item %INC
-The hash %INC contains entries for each filename that has
-been included via C<do> or C<require>. The key is the filename you
-specified, and the value is the location of the file actually found.
-The C<require> command uses this array to determine whether a given file
-has already been included.
+The hash %INC contains entries for each filename included via the
+C<do>, C<require>, or C<use> operators. The key is the filename
+you specified (with module names converted to pathnames), and the
+value is the location of the file found. The C<require>
+operator uses this hash to determine whether a particular file has
+already been included.
+
+If the file was loaded via a hook (e.g. a subroutine reference, see
+L<perlfunc/require> for a description of these hooks), this hook is
+by default inserted into %INC in place of a filename. Note, however,
+that the hook may have set the %INC entry by itself to provide some more
+specific info.
+
+=item %ENV
-=item %ENV $ENV{expr}
+=item $ENV{expr}
The hash %ENV contains your current environment. Setting a
-value in C<ENV> changes the environment for child processes.
+value in C<ENV> changes the environment for any child processes
+you subsequently fork() off.
-=item %SIG $SIG{expr}
+=item %SIG
-The hash %SIG is used to set signal handlers for various
-signals. Example:
+=item $SIG{expr}
+
+The hash %SIG contains signal handlers for signals. For example:
sub handler { # 1st argument is signal name
my($sig) = @_;
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
- $SIG{'INT'} = 'DEFAULT'; # restore default action
+ $SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of C<'IGNORE'> usually has the effect of ignoring the
signal, except for the C<CHLD> signal. See L<perlipc> for more about
this special case.
-The %SIG array contains values for only the signals actually set within
-the Perl script. Here are some other examples:
+Here are some other examples:
- $SIG{"PIPE"} = Plumber; # SCARY!!
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
+ $SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
-The one marked scary is problematic because it's a bareword, which means
-sometimes it's a string representing the function, and sometimes it's
-going to call the subroutine call right then and there! Best to be sure
-and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
+Be sure not to use a bareword as the name of a signal handler,
+lest you inadvertently call it.
If your system has the sigaction() function then signal handlers are
installed using it. This means you get reliable signal handling. If
your system has the SA_RESTART flag it is used when signals handlers are
-installed. This means that system calls for which it is supported
+installed. This means that system calls for which restarting is supported
continue rather than returning when a signal arrives. If you want your
system calls to be interrupted by signal delivery then do something like
this:
The C<__DIE__> handler is explicitly disabled during the call, so that you
can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
-Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
-blocks/strings. See L<perlfunc/die> and L<perlvar/$^S> for how to
-circumvent this.
-
-Note that C<__DIE__>/C<__WARN__> handlers are very special in one
-respect: they may be called to report (probable) errors found by the
-parser. In such a case the parser may be in inconsistent state, so
-any attempt to evaluate Perl code from such a handler will probably
-result in a segfault. This means that calls which result/may-result
-in parsing Perl should be used with extreme causion, like this:
+Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
+even inside an eval(). Do not use this to rewrite a pending exception
+in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
+This strange action at a distance may be fixed in a future release
+so that C<$SIG{__DIE__}> is only called if your program is about
+to exit, as was the original intent. Any other use is deprecated.
+
+C<__DIE__>/C<__WARN__> handlers are very special in one respect:
+they may be called to report (probable) errors found by the parser.
+In such a case the parser may be in inconsistent state, so any
+attempt to evaluate Perl code from such a handler will probably
+result in a segfault. This means that warnings or errors that
+result from parsing Perl should be used with extreme caution, like
+this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
Carp was available. The third line will be executed only if Carp was
not available.
-See L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval> for
-additional info.
+See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
+L<warnings> for additional information.
=back
=head2 Error Indicators
-The variables L<$@>, L<$!>, L<$^E>, and L<$?> contain information about
-different types of error conditions that may appear during execution of
-Perl script. The variables are shown ordered by the "distance" between
-the subsystem which reported the error and the Perl process, and
-correspond to errors detected by the Perl interpreter, C library,
-operating system, or an external program, respectively.
+The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
+about different types of error conditions that may appear during
+execution of a Perl program. The variables are shown ordered by
+the "distance" between the subsystem which reported the error and
+the Perl process. They correspond to errors detected by the Perl
+interpreter, C library, operating system, or an external program,
+respectively.
To illustrate the differences between these variables, consider the
-following Perl expression:
+following Perl expression, which uses a single-quoted string:
- eval '
- open PIPE, "/cdrom/install |";
- @res = <PIPE>;
- close PIPE or die "bad pipe: $?, $!";
- ';
+ eval q{
+ open my $pipe, "/cdrom/install |" or die $!;
+ my @res = <$pipe>;
+ close $pipe or die "bad pipe: $?, $!";
+ };
After execution of this statement all 4 variables may have been set.
-$@ is set if the string to be C<eval>-ed did not compile (this may happen if
-C<open> or C<close> were imported with bad prototypes), or if Perl
-code executed during evaluation die()d (either implicitly, say,
-if C<open> was imported from module L<Fatal>, or the C<die> after
-C<close> was triggered). In these cases the value of $@ is the compile
-error, or C<Fatal> error (which will interpolate C<$!>!), or the argument
-to C<die> (which will interpolate C<$!> and C<$?>!).
-
-When the above expression is executed, open(), C<<PIPEE<gt>>, and C<close>
-are translated to C run-time library calls. $! is set if one of these
-calls fails. The value is a symbolic indicator chosen by the C run-time
-library, say C<No such file or directory>.
-
-On some systems the above C library calls are further translated
-to calls to the kernel. The kernel may have set more verbose error
-indicator that one of the handful of standard C errors. In such cases $^E
-contains this verbose error indicator, which may be, say, C<CDROM tray not
-closed>. On systems where C library calls are identical to system calls
-$^E is a duplicate of $!.
-
-Finally, $? may be set to non-C<0> value if the external program
-C</cdrom/install> fails. Upper bits of the particular value may reflect
-specific error conditions encountered by this program (this is
-program-dependent), lower-bits reflect mode of failure (segfault, completion,
-etc.). Note that in contrast to $@, $!, and $^E, which are set only
-if error condition is detected, the variable $? is set on each C<wait> or
-pipe C<close>, overwriting the old value.
-
-For more details, see the individual descriptions at L<$@>, L<$!>, L<$^E>,
-and L<$?>.
+C<$@> is set if the string to be C<eval>-ed did not compile (this
+may happen if C<open> or C<close> were imported with bad prototypes),
+or if Perl code executed during evaluation die()d . In these cases
+the value of $@ is the compile error, or the argument to C<die>
+(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
+though.)
+
+When the eval() expression above is executed, open(), C<< <PIPE> >>,
+and C<close> are translated to calls in the C run-time library and
+thence to the operating system kernel. C<$!> is set to the C library's
+C<errno> if one of these calls fails.
+
+Under a few operating systems, C<$^E> may contain a more verbose
+error indicator, such as in this case, "CDROM tray not closed."
+Systems that do not support extended error messages leave C<$^E>
+the same as C<$!>.
+
+Finally, C<$?> may be set to non-0 value if the external program
+F</cdrom/install> fails. The upper eight bits reflect specific
+error conditions encountered by the program (the program's exit()
+value). The lower eight bits reflect mode of failure, like signal
+death and core dump information See wait(2) for details. In
+contrast to C<$!> and C<$^E>, which are set only if error condition
+is detected, the variable C<$?> is set on each C<wait> or pipe
+C<close>, overwriting the old value. This is more like C<$@>, which
+on every eval() is always set on failure and cleared on success.
+
+For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
+and C<$?>.
+
+=head2 Technical Note on the Syntax of Variable Names
+
+Variable names in Perl can have several formats. Usually, they
+must begin with a letter or underscore, in which case they can be
+arbitrarily long (up to an internal limit of 251 characters) and
+may contain letters, digits, underscores, or the special sequence
+C<::> or C<'>. In this case, the part before the last C<::> or
+C<'> is taken to be a I<package qualifier>; see L<perlmod>.
+
+Perl variable names may also be a sequence of digits or a single
+punctuation or control character. These names are all reserved for
+special uses by Perl; for example, the all-digits names are used
+to hold data captured by backreferences after a regular expression
+match. Perl has a special syntax for the single-control-character
+names: It understands C<^X> (caret C<X>) to mean the control-C<X>
+character. For example, the notation C<$^W> (dollar-sign caret
+C<W>) is the scalar variable whose name is the single character
+control-C<W>. This is better than typing a literal control-C<W>
+into your program.
+
+Finally, new in Perl 5.6, Perl variable names may be alphanumeric
+strings that begin with control characters (or better yet, a caret).
+These variables must be written in the form C<${^Foo}>; the braces
+are not optional. C<${^Foo}> denotes the scalar variable whose
+name is a control-C<F> followed by two C<o>'s. These variables are
+reserved for future special uses by Perl, except for the ones that
+begin with C<^_> (control-underscore or caret-underscore). No
+control-character name that begins with C<^_> will acquire a special
+meaning in any future version of Perl; such names may therefore be
+used safely in programs. C<$^_> itself, however, I<is> reserved.
+
+Perl identifiers that begin with digits, control characters, or
+punctuation characters are exempt from the effects of the C<package>
+declaration and are always forced to be in package C<main>. A few
+other names are also exempt:
+
+ ENV STDIN
+ INC STDOUT
+ ARGV STDERR
+ ARGVOUT
+ SIG
+
+In particular, the new special C<${^_XYZ}> variables are always taken
+to be in package C<main>, regardless of any C<package> declarations
+presently in scope.
+
+=head1 BUGS
+
+Due to an unfortunate accident of Perl's implementation, C<use
+English> imposes a considerable performance penalty on all regular
+expression matches in a program, regardless of whether they occur
+in the scope of C<use English>. For that reason, saying C<use
+English> in libraries is strongly discouraged. See the
+Devel::SawAmpersand module documentation from CPAN
+(http://www.cpan.org/modules/by-module/Devel/)
+for more information.
+
+Having to even think about the C<$^S> variable in your exception
+handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
+invites grievous and difficult to track down errors. Avoid it
+and use an C<END{}> or CORE::GLOBAL::die override instead.
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