use strict;
use warnings;

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;

# seeking for a function like:
# list_of_derived_classes ('Element');
# which returns -> 'Circle', 'Square', 'Line'