$self= new $breed;
. . . 
bless $self,$breed;
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I don't think this will do what you want. This will re-bless the reference into the dog package, which means that if you asked for a 'cocker' breed, any methods that dog::cocker overrides won't be called. Instead, the methods in dog will be called. Update: Read the code wrong--ignore this pargraph.

Which is really unfortunate, since it could be quite useful to override the bark method in your final example. Instead of checking if this instance has a voice attribute that is true, the subclass would already know and Do The Right Thing. That's where polymorphism starts becoming powerful.