Probably because Perl's OO is a hack on top of package namespaces. Constructed at a time when references were still really new in Perl, and nobody knew how useful they would turn out to be or how far they would go.

And the idea of a class as an object is not part of Perl. Indeed there are many Perl programmers who think it a ood idea to make what I would say should be class methods into object methods. (Yes, I am referring to the testing in new whether you are passed something blessed...)