See http://search.cpan.org/~ether/Moose-2.1403/lib/Moose/Manual/... and http://search.cpan.org/~ether/Moose-2.1403/lib/Moose/Manual....
Competing type systems, competing Object systems, and enough internal run-time rewriting to do whatever it is you want.
See also http://search.cpan.org/~smueller/Filter-Simple-0.91/lib/Filt... for a simple rewriting filter, and the Acme namespace for all kinds of fancy experiments.