This module provides those interfaces for earlier versions of Perl (back to 5.6.0 anyways).
It is a harmless no-op to use this module on 5.9.5+. That is to say, code which properly uses MRO::Compat will work unmodified on both older Perls and 5.9.5+.
If you're writing a piece of software that would like to use the parts of 5.9.5+'s mro:: interfaces that are supported here, and you want compatibility with older Perls, this is the module for you.