How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regexp}?

Passing Variables and Functions

Regular variables and functions are quite easy: just pass in a reference to an existing or anonymous variable or function:

    func( \$some_scalar );

    func( \$some_array );
    func( [ 1 .. 10 ]   );

    func( \%some_hash   );
    func( { this => 10, that => 20 }   );

    func( \&some_func   );
    func( sub { $_[0] ** $_[1] }   );

Passing Filehandles

To create filehandles you can pass to subroutines, you can use *FH or \*FH notation (``typeglobs'' - see the perldata manpage for more information), or create filehandles dynamically using the old FileHandle or the new IO::File modules, both part of the standard Perl distribution.

    use Fcntl;
    use IO::File;
    my $fh = new IO::File $filename, O_WRONLY|O_APPEND;
                or die "Can't append to $filename: $!";
    func($fh);

Passing Regexps

To pass regexps around, you'll need to either use one of the highly experimental regular expression modules from CPAN (Nick Ing-Simmons's Regexp or Ilya Zakharevich's Devel::Regexp), pass around strings and use an exception-trapping eval, or else be be very, very clever. Here's an example of how to pass in a string to be regexp compared:

    sub compare($$) {
        my ($val1, $regexp) = @_;
        my $retval = eval { $val =~ /$regexp/ };
        die if $@;
        return $retval;
    }

    $match = compare("old McDonald", q/d.*D/);

Make sure you never say something like this:

    return eval "\$val =~ /$regexp/";   # WRONG

or someone can sneak shell escapes into the regexp due to the double interpolation of the eval and the double-quoted string. For example:

    $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';

    eval "\$string =~ /$pattern_of_evil/";

Those preferring to be very, very clever might see the O'Reilly book, Mastering Regular Expressions, by Jeffrey Friedl. Page 273's Build_MatchMany_Function() is particularly interesting. A complete citation of this book is given in the perlfaq2 manpage.

Passing Methods

To pass an object method into a subroutine, you can do this:

    call_a_lot(10, $some_obj, "methname")
    sub call_a_lot {
        my ($count, $widget, $trick) = @_;
        for (my $i = 0; $i < $count; $i++) {
            $widget->$trick();
        }
    }

or you can use a closure to bundle up the object and its method call and arguments:

    my $whatnot =  sub { $some_obj->obfuscate(@args) };
    func($whatnot);
    sub func {
        my $code = shift;
        &$code();
    }

You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution).