sub quack(RubberDuck $self) { ... }
sub quack(Mallard $self)    { ... }

##</code><code>##

package RubberDuck;

sub new { bless {}, shift }

sub make_noise { print "Plug yer ears!\n"; $_[0]->quack(); }
sub quack { print "Squeak!\n" }

sub DESTROY { print "RubberDuck goes back in the toybox.\n" }

package Mallard;

our @ISA = qw(RubberDuck);
sub new { bless {}, shift }

sub quack { print "Quack!\n" }

sub DESTROY { print "Mallard flies away.\n" }

package main;

for my $duck_class (qw( RubberDuck Mallard ))
{
    my $duck = $duck_class->new();
    $duck->make_noise();
}