package SwimmingFlyingThing;
use AbstractClass qw(swim fly);

##</code><code>##

pacakage AbstractBird;
use SwimmingFlyingThing;
use AbstractClass;
@ISA = 'SimmingFlyingThing';
sub fly { print "Flap Flap Flap" }

##</code><code>##

package Duck;
@ISA = 'AbstractBird';
sub swim { print "Paddle paddle paddle" }
sub new { ... }