#!/usr/bin/perl -w
use strict;

package Bar;
use base 'IO::Handle';

package Element;
 sub new {
 my $type = shift;
 my $self = {};
 bless $self, $type;
 }

package Circle;
 use vars qw(@ISA);
 @ISA = qw( Element );

package Square;
 use vars qw(@ISA);
 @ISA = qw( Element );

package Line;
 use vars qw(@ISA);
 @ISA = qw( Element );

package main;

sub list_of_derived_classes {
    my $class = shift;
    my @pkg_keys;
    {
        no strict 'refs';
        @pkg_keys = grep { /::$/ }  keys %{__PACKAGE__.'::'};
    }
    my @result;
    for (@pkg_keys) {
        no strict 'refs';
        if (defined ${$_}{ISA}) {
            push @result, $_ if grep {/^$class$/} @{${$_}{ISA}};
        }
    }
    s/::$// for @result;
    @result;
}
for my $class (qw(Element IO::Handle)) {
    print "$class\: ",join(', ', list_of_derived_classes($class)),$/;
}
__END__
Element: Circle, Line, Square
IO::Handle: Bar