perlsyn
(1)
Name
perlsyn - Perl syntax
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLSYN(1)
NAME
perlsyn - Perl syntax
DESCRIPTION
A Perl program consists of a sequence of declarations and
statements which run from the top to the bottom. Loops,
subroutines and other control structures allow you to jump
around within the code.
Perl is a free-form language, you can format and indent it
however you like. Whitespace mostly serves to separate
tokens, unlike languages like Python where it is an
important part of the syntax.
Many of Perl's syntactic elements are optional. Rather than
requiring you to put parentheses around every function call
and declare every variable, you can often leave such
explicit elements off and Perl will figure out what you
meant. This is known as Do What I Mean, abbreviated DWIM.
It allows programmers to be lazy and to code in a style with
which they are comfortable.
Perl borrows syntax and concepts from many languages: awk,
sed, C, Bourne Shell, Smalltalk, Lisp and even English.
Other languages have borrowed syntax from Perl, particularly
its regular expression extensions. So if you have
programmed in another language you will see familiar pieces
in Perl. They often work the same, but see perltrap for
information about how they differ.
Declarations
The only things you need to declare in Perl are report
formats and subroutines (and sometimes not even
subroutines). A variable holds the undefined value
("undef") until it has been assigned a defined value, which
is anything other than "undef". When used as a number,
"undef" is treated as 0; when used as a string, it is
treated as the empty string, ""; and when used as a
reference that isn't being assigned to, it is treated as an
error. If you enable warnings, you'll be notified of an
uninitialized value whenever you treat "undef" as a string
or a number. Well, usually. Boolean contexts, such as:
my $a;
if ($a) {}
are exempt from warnings (because they care about truth
rather than definedness). Operators such as "++", "--",
"+=", "-=", and ".=", that operate on undefined left values
such as:
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my $a;
$a++;
are also always exempt from such warnings.
A declaration can be put anywhere a statement can, but has
no effect on the execution of the primary sequence of
statements--declarations all take effect at compile time.
Typically all the declarations are put at the beginning or
the end of the script. However, if you're using lexically-
scoped private variables created with "my()", you'll have to
make sure your format or subroutine definition is within the
same block scope as the my if you expect to be able to
access those private variables.
Declaring a subroutine allows a subroutine name to be used
as if it were a list operator from that point forward in the
program. You can declare a subroutine without defining it
by saying "sub name", thus:
sub myname;
$me = myname $0 or die "can't get myname";
Note that myname() functions as a list operator, not as a
unary operator; so be careful to use "or" instead of "||" in
this case. However, if you were to declare the subroutine
as "sub myname ($)", then "myname" would function as a unary
operator, so either "or" or "||" would work.
Subroutines declarations can also be loaded up with the
"require" statement or both loaded and imported into your
namespace with a "use" statement. See perlmod for details
on this.
A statement sequence may contain declarations of lexically-
scoped variables, but apart from declaring a variable name,
the declaration acts like an ordinary statement, and is
elaborated within the sequence of statements as if it were
an ordinary statement. That means it actually has both
compile-time and run-time effects.
Comments
Text from a "#" character until the end of the line is a
comment, and is ignored. Exceptions include "#" inside a
string or regular expression.
Simple Statements
The only kind of simple statement is an expression evaluated
for its side effects. Every simple statement must be
terminated with a semicolon, unless it is the final
statement in a block, in which case the semicolon is
optional. (A semicolon is still encouraged if the block
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takes up more than one line, because you may eventually add
another line.) Note that there are some operators like
"eval {}" and "do {}" that look like compound statements,
but aren't (they're just TERMs in an expression), and thus
need an explicit termination if used as the last item in a
statement.
Truth and Falsehood
The number 0, the strings '0' and '', the empty list "()",
and "undef" are all false in a boolean context. All other
values are true. Negation of a true value by "!" or "not"
returns a special false value. When evaluated as a string
it is treated as '', but as a number, it is treated as 0.
Statement Modifiers
Any simple statement may optionally be followed by a SINGLE
modifier, just before the terminating semicolon (or block
ending). The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
when EXPR
for LIST
foreach LIST
The "EXPR" following the modifier is referred to as the
"condition". Its truth or falsehood determines how the
modifier will behave.
"if" executes the statement once if and only if the
condition is true. "unless" is the opposite, it executes
the statement unless the condition is true (i.e., if the
condition is false).
print "Basset hounds got long ears" if length $ear >= 10;
go_outside() and play() unless $is_raining;
"when" executes the statement when $_ smart matches "EXPR",
and then either "break"s out if it's enclosed in a "given"
scope or skips to the "next" element when it lies directly
inside a "for" loop. See also "Switch statements".
given ($something) {
$abc = 1 when /^abc/;
$just_a = 1 when /^a/;
$other = 1;
}
for (@names) {
admin($_) when [ qw/Alice Bob/ ];
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regular($_) when [ qw/Chris David Ellen/ ];
}
The "foreach" modifier is an iterator: it executes the
statement once for each item in the LIST (with $_ aliased to
each item in turn).
print "Hello $_!\n" foreach qw(world Dolly nurse);
"while" repeats the statement while the condition is true.
"until" does the opposite, it repeats the statement until
the condition is true (or while the condition is false):
# Both of these count from 0 to 10.
print $i++ while $i <= 10;
print $j++ until $j > 10;
The "while" and "until" modifiers have the usual ""while"
loop" semantics (conditional evaluated first), except when
applied to a "do"-BLOCK (or to the deprecated
"do"-SUBROUTINE statement), in which case the block executes
once before the conditional is evaluated. This is so that
you can write loops like:
do {
$line = <STDIN>;
...
} until $line eq ".\n";
See "do" in perlfunc. Note also that the loop control
statements described later will NOT work in this construct,
because modifiers don't take loop labels. Sorry. You can
always put another block inside of it (for "next") or around
it (for "last") to do that sort of thing. For "next", just
double the braces:
do {{
next if $x == $y;
# do something here
}} until $x++ > $z;
For "last", you have to be more elaborate:
LOOP: {
do {
last if $x = $y**2;
# do something here
} while $x++ <= $z;
}
NOTE: The behaviour of a "my" statement modified with a
statement modifier conditional or loop construct (e.g. "my
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$x if ...") is undefined. The value of the "my" variable
may be "undef", any previously assigned value, or possibly
anything else. Don't rely on it. Future versions of perl
might do something different from the version of perl you
try it out on. Here be dragons.
Compound Statements
In Perl, a sequence of statements that defines a scope is
called a block. Sometimes a block is delimited by the file
containing it (in the case of a required file, or the
program as a whole), and sometimes a block is delimited by
the extent of a string (in the case of an eval).
But generally, a block is delimited by curly brackets, also
known as braces. We will call this syntactic construct a
BLOCK.
The following compound statements may be used to control
flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
unless (EXPR) BLOCK
unless (EXPR) BLOCK else BLOCK
unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL until (EXPR) BLOCK
LABEL until (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
LABEL foreach VAR (LIST) BLOCK continue BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms
of BLOCKs, not statements. This means that the curly
brackets are required--no dangling statements allowed. If
you want to write conditionals without curly brackets there
are several other ways to do it. The following all do the
same thing:
if (!open(FOO)) { die "Can't open $FOO: $!"; }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
The "if" statement is straightforward. Because BLOCKs are
always bounded by curly brackets, there is never any
ambiguity about which "if" an "else" goes with. If you use
"unless" in place of "if", the sense of the test is
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reversed. Like "if", "unless" can be followed by "else".
"unless" can even be followed by one or more "elsif"
statements, though you may want to think twice before using
that particular language construct, as everyone reading your
code will have to think at least twice before they can
understand what's going on.
The "while" statement executes the block as long as the
expression is true. The "until" statement executes the
block as long as the expression is false. The LABEL is
optional, and if present, consists of an identifier followed
by a colon. The LABEL identifies the loop for the loop
control statements "next", "last", and "redo". If the LABEL
is omitted, the loop control statement refers to the
innermost enclosing loop. This may include dynamically
looking back your call-stack at run time to find the LABEL.
Such desperate behavior triggers a warning if you use the
"use warnings" pragma or the -w flag.
If there is a "continue" BLOCK, it is always executed just
before the conditional is about to be evaluated again. Thus
it can be used to increment a loop variable, even when the
loop has been continued via the "next" statement.
Extension modules can also hook into the Perl parser to
define new kinds of compound statement. These are
introduced by a keyword which the extension recognises, and
the syntax following the keyword is defined entirely by the
extension. If you are an implementor, see
"PL_keyword_plugin" in perlapi for the mechanism. If you
are using such a module, see the module's documentation for
details of the syntax that it defines.
Loop Control
The "next" command starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The "last" command immediately exits the loop in question.
The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The "redo" command restarts the loop block without
evaluating the conditional again. The "continue" block, if
any, is not executed. This command is normally used by
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programs that want to lie to themselves about what was just
input.
For example, when processing a file like /etc/termcap. If
your input lines might end in backslashes to indicate
continuation, you want to skip ahead and get the next
record.
while (<>) {
chomp;
if (s/\\$//) {
$_ .= <>;
redo unless eof();
}
# now process $_
}
which is Perl short-hand for the more explicitly written
version:
LINE: while (defined($line = <ARGV>)) {
chomp($line);
if ($line =~ s/\\$//) {
$line .= <ARGV>;
redo LINE unless eof(); # not eof(ARGV)!
}
# now process $line
}
Note that if there were a "continue" block on the above
code, it would get executed only on lines discarded by the
regex (since redo skips the continue block). A continue
block is often used to reset line counters or "?pat?" one-
time matches:
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
?(fred)? && s//WILMA $1 WILMA/;
?(barney)? && s//BETTY $1 BETTY/;
?(homer)? && s//MARGE $1 MARGE/;
} continue {
print "$ARGV $.: $_";
close ARGV if eof(); # reset $.
reset if eof(); # reset ?pat?
}
If the word "while" is replaced by the word "until", the
sense of the test is reversed, but the conditional is still
tested before the first iteration.
The loop control statements don't work in an "if" or
"unless", since they aren't loops. You can double the
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braces to make them such, though.
if (/pattern/) {{
last if /fred/;
next if /barney/; # same effect as "last", but doesn't document as well
# do something here
}}
This is caused by the fact that a block by itself acts as a
loop that executes once, see "Basic BLOCKs".
The form "while/if BLOCK BLOCK", available in Perl 4, is no
longer available. Replace any occurrence of "if BLOCK" by
"if (do BLOCK)".
For Loops
Perl's C-style "for" loop works like the corresponding
"while" loop; that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
There is one minor difference: if variables are declared
with "my" in the initialization section of the "for", the
lexical scope of those variables is exactly the "for" loop
(the body of the loop and the control sections).
Besides the normal array index looping, "for" can lend
itself to many other interesting applications. Here's one
that avoids the problem you get into if you explicitly test
for end-of-file on an interactive file descriptor causing
your program to appear to hang.
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
# do something
}
Using "readline" (or the operator form, "<EXPR>") as the
conditional of a "for" loop is shorthand for the following.
This behaviour is the same as a "while" loop conditional.
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for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
# do something
}
Foreach Loops
The "foreach" loop iterates over a normal list value and
sets the variable VAR to be each element of the list in
turn. If the variable is preceded with the keyword "my",
then it is lexically scoped, and is therefore visible only
within the loop. Otherwise, the variable is implicitly
local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with
"my", it uses that variable instead of the global one, but
it's still localized to the loop. This implicit
localisation occurs only in a "foreach" loop.
The "foreach" keyword is actually a synonym for the "for"
keyword, so you can use "foreach" for readability or "for"
for brevity. (Or because the Bourne shell is more familiar
to you than csh, so writing "for" comes more naturally.) If
VAR is omitted, $_ is set to each value.
If any element of LIST is an lvalue, you can modify it by
modifying VAR inside the loop. Conversely, if any element
of LIST is NOT an lvalue, any attempt to modify that element
will fail. In other words, the "foreach" loop index
variable is an implicit alias for each item in the list that
you're looping over.
If any part of LIST is an array, "foreach" will get very
confused if you add or remove elements within the loop body,
for example with "splice". So don't do that.
"foreach" probably won't do what you expect if VAR is a tied
or other special variable. Don't do that either.
Examples:
for (@ary) { s/foo/bar/ }
for my $elem (@elements) {
$elem *= 2;
}
for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
print $count, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
print "Item: $item\n";
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}
Here's how a C programmer might code up a particular
algorithm in Perl:
for (my $i = 0; $i < @ary1; $i++) {
for (my $j = 0; $j < @ary2; $j++) {
if ($ary1[$i] > $ary2[$j]) {
last; # can't go to outer :-(
}
$ary1[$i] += $ary2[$j];
}
# this is where that last takes me
}
Whereas here's how a Perl programmer more comfortable with
the idiom might do it:
OUTER: for my $wid (@ary1) {
INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and
faster. It's cleaner because it's less noisy. It's safer
because if code gets added between the inner and outer loops
later on, the new code won't be accidentally executed. The
"next" explicitly iterates the other loop rather than merely
terminating the inner one. And it's faster because Perl
executes a "foreach" statement more rapidly than it would
the equivalent "for" loop.
Basic BLOCKs
A BLOCK by itself (labeled or not) is semantically
equivalent to a loop that executes once. Thus you can use
any of the loop control statements in it to leave or restart
the block. (Note that this is NOT true in "eval{}",
"sub{}", or contrary to popular belief "do{}" blocks, which
do NOT count as loops.) The "continue" block is optional.
The BLOCK construct can be used to emulate case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
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Such constructs are quite frequently used, because older
versions of Perl had no official "switch" statement.
Switch statements
Starting from Perl 5.10, you can say
use feature "switch";
which enables a switch feature that is closely based on the
Perl 6 proposal.
The keywords "given" and "when" are analogous to "switch"
and "case" in other languages, so the code above could be
written as
given($_) {
when (/^abc/) { $abc = 1; }
when (/^def/) { $def = 1; }
when (/^xyz/) { $xyz = 1; }
default { $nothing = 1; }
}
This construct is very flexible and powerful. For example:
use feature ":5.10";
given($foo) {
when (undef) {
say '$foo is undefined';
}
when ("foo") {
say '$foo is the string "foo"';
}
when ([1,3,5,7,9]) {
say '$foo is an odd digit';
continue; # Fall through
}
when ($_ < 100) {
say '$foo is numerically less than 100';
}
when (\&complicated_check) {
say 'a complicated check for $foo is true';
}
default {
die q(I don't know what to do with $foo);
}
}
"given(EXPR)" will assign the value of EXPR to $_ within the
lexical scope of the block, so it's similar to
do { my $_ = EXPR; ... }
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except that the block is automatically broken out of by a
successful "when" or an explicit "break".
Most of the power comes from implicit smart matching:
when($foo)
is exactly equivalent to
when($_ ~~ $foo)
Most of the time, "when(EXPR)" is treated as an implicit
smart match of $_, i.e. "$_ ~~ EXPR". (See "Smart matching
in detail" for more information on smart matching.) But when
EXPR is one of the below exceptional cases, it is used
directly as a boolean:
o a subroutine or method call
o a regular expression match, i.e. "/REGEX/" or "$foo =~
/REGEX/", or a negated regular expression match
("!/REGEX/" or "$foo !~ /REGEX/").
o a comparison such as "$_ < 10" or "$x eq "abc"" (or of
course "$_ ~~ $c")
o "defined(...)", "exists(...)", or "eof(...)"
o a negated expression "!(...)" or "not (...)", or a
logical exclusive-or "(...) xor (...)".
o a filetest operator, with the exception of "-s", "-M",
"-A", and "-C", that return numerical values, not
boolean ones.
o the ".." and "..." flip-flop operators.
In those cases the value of EXPR is used directly as a
boolean.
Furthermore:
o If EXPR is "... && ..." or "... and ...", the test is
applied recursively to both arguments. If both arguments
pass the test, then the argument is treated as boolean.
o If EXPR is "... || ...", "... // ..." or "... or ...",
the test is applied recursively to the first argument.
These rules look complicated, but usually they will do what
you want. For example you could write:
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when (/^\d+$/ && $_ < 75) { ... }
Another useful shortcut is that, if you use a literal array
or hash as the argument to "given", it is turned into a
reference. So "given(@foo)" is the same as "given(\@foo)",
for example.
"default" behaves exactly like "when(1 == 1)", which is to
say that it always matches.
Breaking out
You can use the "break" keyword to break out of the
enclosing "given" block. Every "when" block is implicitly
ended with a "break".
Fall-through
You can use the "continue" keyword to fall through from one
case to the next:
given($foo) {
when (/x/) { say '$foo contains an x'; continue }
when (/y/) { say '$foo contains a y' }
default { say '$foo does not contain a y' }
}
Switching in a loop
Instead of using "given()", you can use a "foreach()" loop.
For example, here's one way to count how many times a
particular string occurs in an array:
my $count = 0;
for (@array) {
when ("foo") { ++$count }
}
print "\@array contains $count copies of 'foo'\n";
At the end of all "when" blocks, there is an implicit
"next". You can override that with an explicit "last" if
you're only interested in the first match.
This doesn't work if you explicitly specify a loop variable,
as in "for $item (@array)". You have to use the default
variable $_. (You can use "for my $_ (@array)".)
Smart matching in detail
The behaviour of a smart match depends on what type of thing
its arguments are. The behaviour is determined by the
following table: the first row that applies determines the
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match behaviour (which is thus mostly determined by the type
of the right operand). Note that the smart match implicitly
dereferences any non-blessed hash or array ref, so the
"Hash" and "Array" entries apply in those cases. (For
blessed references, the "Object" entries apply.)
Note that the "Matching Code" column is not always an exact
rendition. For example, the smart match operator short-
circuits whenever possible, but "grep" does not.
$a $b Type of Match Implied Matching Code
====== ===== ===================== =============
Any undef undefined !defined $a
Any Object invokes ~~ overloading on $object, or dies
Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
Any CodeRef scalar sub truth $b->($a)
Hash Hash hash keys identical (every key is found in both hashes)
Array Hash hash keys intersection grep { exists $b->{$_} } @$a
Regex Hash hash key grep grep /$a/, keys %$b
undef Hash always false (undef can't be a key)
Any Hash hash entry existence exists $b->{$a}
Hash Array hash keys intersection grep { exists $a->{$_} } @$b
Array Array arrays are comparable[2]
Regex Array array grep grep /$a/, @$b
undef Array array contains undef grep !defined, @$b
Any Array match against an array element[3]
grep $a ~~ $_, @$b
Hash Regex hash key grep grep /$b/, keys %$a
Array Regex array grep grep /$b/, @$a
Any Regex pattern match $a =~ /$b/
Object Any invokes ~~ overloading on $object, or falls back:
Any Num numeric equality $a == $b
Num numish[4] numeric equality $a == $b
undef Any undefined !defined($b)
Any Any string equality $a eq $b
1 - empty hashes or arrays will match.
2 - that is, each element smart-matches the element of same index in the
other array. [3]
3 - If a circular reference is found, we fall back to referential equality.
4 - either a real number, or a string that looks like a number
Custom matching via overloading
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You can change the way that an object is matched by
overloading the "~~" operator. This may alter the usual
smart match semantics.
It should be noted that "~~" will refuse to work on objects
that don't overload it (in order to avoid relying on the
object's underlying structure).
Note also that smart match's matching rules take precedence
over overloading, so if $obj has smart match overloading,
then
$obj ~~ X
will not automatically invoke the overload method with X as
an argument; instead the table above is consulted as normal,
and based in the type of X, overloading may or may not be
invoked.
See overload.
Differences from Perl 6
The Perl 5 smart match and "given"/"when" constructs are not
absolutely identical to their Perl 6 analogues. The most
visible difference is that, in Perl 5, parentheses are
required around the argument to "given()" and "when()"
(except when this last one is used as a statement modifier).
Parentheses in Perl 6 are always optional in a control
construct such as "if()", "while()", or "when()"; they can't
be made optional in Perl 5 without a great deal of potential
confusion, because Perl 5 would parse the expression
given $foo {
...
}
as though the argument to "given" were an element of the
hash %foo, interpreting the braces as hash-element syntax.
The table of smart matches is not identical to that proposed
by the Perl 6 specification, mainly due to the differences
between Perl 6's and Perl 5's data models.
In Perl 6, "when()" will always do an implicit smart match
with its argument, whilst it is convenient in Perl 5 to
suppress this implicit smart match in certain situations, as
documented above. (The difference is largely because Perl 5
does not, even internally, have a boolean type.)
Goto
Although not for the faint of heart, Perl does support a
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"goto" statement. There are three forms: "goto"-LABEL,
"goto"-EXPR, and "goto"-&NAME. A loop's LABEL is not
actually a valid target for a "goto"; it's just the name of
the loop.
The "goto"-LABEL form finds the statement labeled with LABEL
and resumes execution there. It may not be used to go into
any construct that requires initialization, such as a
subroutine or a "foreach" loop. It also can't be used to go
into a construct that is optimized away. It can be used to
go almost anywhere else within the dynamic scope, including
out of subroutines, but it's usually better to use some
other construct such as "last" or "die". The author of Perl
has never felt the need to use this form of "goto" (in Perl,
that is--C is another matter).
The "goto"-EXPR form expects a label name, whose scope will
be resolved dynamically. This allows for computed "goto"s
per FORTRAN, but isn't necessarily recommended if you're
optimizing for maintainability:
goto(("FOO", "BAR", "GLARCH")[$i]);
The "goto"-&NAME form is highly magical, and substitutes a
call to the named subroutine for the currently running
subroutine. This is used by "AUTOLOAD()" subroutines that
wish to load another subroutine and then pretend that the
other subroutine had been called in the first place (except
that any modifications to @_ in the current subroutine are
propagated to the other subroutine.) After the "goto", not
even "caller()" will be able to tell that this routine was
called first.
In almost all cases like this, it's usually a far, far
better idea to use the structured control flow mechanisms of
"next", "last", or "redo" instead of resorting to a "goto".
For certain applications, the catch and throw pair of
"eval{}" and die() for exception processing can also be a
prudent approach.
PODs: Embedded Documentation
Perl has a mechanism for intermixing documentation with
source code. While it's expecting the beginning of a new
statement, if the compiler encounters a line that begins
with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and
including a line beginning with "=cut" will be ignored. The
format of the intervening text is described in perlpod.
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This allows you to intermix your source code and your
documentation text freely, as in
=item snazzle($)
The snazzle() function will behave in the most spectacular
form that you can possibly imagine, not even excepting
cybernetic pyrotechnics.
=cut back to the compiler, nuff of this pod stuff!
sub snazzle($) {
my $thingie = shift;
.........
}
Note that pod translators should look at only paragraphs
beginning with a pod directive (it makes parsing easier),
whereas the compiler actually knows to look for pod escapes
even in the middle of a paragraph. This means that the
following secret stuff will be ignored by both the compiler
and the translators.
$a=3;
=secret stuff
warn "Neither POD nor CODE!?"
=cut back
print "got $a\n";
You probably shouldn't rely upon the "warn()" being podded
out forever. Not all pod translators are well-behaved in
this regard, and perhaps the compiler will become pickier.
One may also use pod directives to quickly comment out a
section of code.
Plain Old Comments (Not!)
Perl can process line directives, much like the C
preprocessor. Using this, one can control Perl's idea of
filenames and line numbers in error or warning messages
(especially for strings that are processed with "eval()").
The syntax for this mechanism is the same as for most C
preprocessors: it matches the regular expression
# example: '# line 42 "new_filename.plx"'
/^\# \s*
line \s+ (\d+) \s*
(?:\s("?)([^"]+)\2)? \s*
$/x
with $1 being the line number for the next line, and $3
being the optional filename (specified with or without
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quotes).
There is a fairly obvious gotcha included with the line
directive: Debuggers and profilers will only show the last
source line to appear at a particular line number in a given
file. Care should be taken not to cause line number
collisions in code you'd like to debug later.
Here are some examples that you should be able to type into
your command shell:
% perl
# line 200 "bzzzt"
# the `#' on the previous line must be the first char on line
die 'foo';
__END__
foo at bzzzt line 201.
% perl
# line 200 "bzzzt"
eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
__END__
foo at - line 2001.
% perl
eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
__END__
foo at foo bar line 200.
% perl
# line 345 "goop"
eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
print $@;
__END__
foo at goop line 345.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
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community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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