Whenever you use "isa" in UNIVERSAL as a function, a kitten using
Test::MockObject dies. Normally, the kittens would be helpless, but if they use
UNIVERSAL::isa (the module whose docs you are reading), the kittens can live
long and prosper.

This module replaces UNIVERSAL::isa with a version that makes sure that, when
called as a function on objects which override isa, isa will call the
appropriate method on those objects

In all other cases, the real UNIVERSAL::isa gets called directly.