I think part of your problem is that Net::ParseWhois::Domain is a package of its own, and you haven't written MyParsewhois::Domain, so the compiler can't find it.

I'm not sure quite how to solve that problem--might it help to create a dummy MyParsewhois::Domain package that had nothing but a pointer to the correct class? (This assumes that you don't need to change any of its methods.)

package MyParsewhois::Domain;
require ParseWhois::Domain;
@ISA = ("ParseWhois::Domain");
__END__
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Alternatively, you could just sneakily declare yourself to be in package Net::Parsewhois, and patch in your own code that way... I believe this is discouraged, however.