perlsyn


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:

           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
       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 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
               $just_a = 1 when /^a/;
               $other  = 1;
           }

           for (@names) {
               admin($_)   when [ qw/Alice Bob/ ];
               regular($_) 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

       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 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
       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 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 "m?pat?" one-time matches:

           # inspired by :1,$g/fred/s//WILMA/
           while (<>) {
               m?(fred)?    && s//WILMA $1 WILMA/;
               m?(barney)?  && s//BETTY $1 BETTY/;
               m?(homer)?   && s//MARGE $1 MARGE/;

       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.

           for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
               # do something
           }

       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";
           }

       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;

   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;
           }

       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';
               }

       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, Perl inspects the operands of the binary boolean operators
       to decide whether to use smart matching for each one by applying the
       above test to the operands:

       o   If EXPR is "... && ..." or "... and ...", the test is applied
           recursively to both operands. If both operands pass the test, then
           the expression is treated as boolean; otherwise, smart matching is
           used.

       o   If EXPR is "... || ...", "... // ..." or "... or ...", the test is
           applied recursively to the first operand (which may be a higher-
           precedence AND operator, for example). If the first operand is to
           use smart matching, then both operands will do so; if it is not,
           then the second argument will not be either.

       These rules look complicated, but usually they will do what you want.
       the other uses smart matching, and that wins.

       Further:

           when ([qw(foo bar)] || /^baz/) { ... }

       will use smart matching (only the first operand is considered), whereas

           when (/^baz/ || [qw(foo bar)]) { ... }

       will test only the regex, which causes both operands to be treated as
       boolean.  Watch out for this one, then, because an arrayref is always a
       true value, which makes it effectively redundant.

       Tautologous boolean operators are still going to be optimized away.
       Don't be tempted to write

           when ('foo' or 'bar') { ... }

       This will optimize down to 'foo', so 'bar' will never be considered
       (even though the rules say to use a smart match on 'foo'). For an
       alternation like this, an array ref will work, because this will
       instigate smart matching:

           when ([qw(foo bar)] { ... }

       This is somewhat equivalent to the C-style switch statement's
       fallthrough functionality (not to be confused with Perl's fallthrough
       functionality - see below), wherein the same block is used for several
       "case" statements.

       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' }
           }


       In both last cases, the last expression is evaluated in the context
       that was applied to the "given" block.

       Note that, unlike "if" and "unless", failed "when" statements always
       evaluate to an empty list.

           my $price = do { given ($item) {
               when ([ 'pear', 'apple' ]) { 1 }
               break when 'vote';      # My vote cannot be bought
               1e10  when /Mona Lisa/;
               'unknown';
           } };

       Currently, "given" blocks can't always be used as proper expressions.
       This may be addressed in a future version of perl.

       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 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

           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

       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
       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 "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.

       This allows you to intermix your source code and your documentation
               .........
           }

       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 almost the same as
       for most C preprocessors: it matches the regular expression

           # example: '# line 42 "new_filename.plx"'
           /^\#   \s*
             line \s+ (\d+)   \s*
             (?:\s("?)([^"]+)\g2)? \s*
            $/x

       with $1 being the line number for the next line, and $3 being the
       optional filename (specified with or without quotes). Note that no
       whitespace may precede the "#", unlike modern C preprocessors.

       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.

           # line 345 "goop"
           eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
           print $@;
           __END__
           foo at goop line 345.



perl v5.14.2                      2011-09-26                        PERLSYN(1)
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