Unfortunately, that solution suffers from the same fundamental problem: a derived class still has to know that $self is a hashref (in this case a blessed one, but that doesn't matter for our purposes). Without using Abigail's inside out objects trick, you cannot implement a subclass in Perl without knowing at least this one detail about the implementation of the superclass - what kind of thing $self is and how to store instance data in it.

Makeshifts last the longest.