package Animal; 

sub new { return bless {},"Animal"; } 
sub speak { };

1; 

package Animal::Dog;
use Animal;
our @ISA = qw/ Animal /;

sub new { return bless {},"Animal::Dog"; } 
sub speak { print "Woof!\n"; } 

1;

package Animal::Cat;
use Animal;
our @ISA= qw/ Animal /;

sub new { reutrn bless {},"Animal::Cat"; } 
sub speak { print "Meow!\n"; } 

1; 

-- ad nauseum --