Polymorphism is a much more general concept, and we skip over that generality by starting with the factory piece first.

See my reply below for why this isn't a good example of polymorphisim anyway. Update: blue_cowdawg's update fix the OP's polymorphisim issue.

I would be interested to see if the factory could be made without using eval, as well...I think it can, especially if the dogs were loaded previously in a more-safe matter.

It can using require $path_to_breed; instead of eval "use $breed;";. But it ammounts to almost the same thing, except that you have to specifiy the path to the breed file instead of the traditional 'module::name' format. As you say, it's also possible to load all the subclasses before anything is called, but that could get very inefficient fast.

My favored solution would be to map a breed via a hash:

my %breeds = (
    cocker   => 'dog::cocker', 
    setter   => 'dog::setter', 
);

sub new 
{
    my $class = shift;
    my $breed = shift;
    
    my $self = { };
    if($breed) {
        my $breed_class = $breeds{$breed};
        eval " use $breed_class ";
        die $@ if $@;
        $self = $breed_class->new();
    }
    else {
        bless $self, $class;
    }

    return $self;
}
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