Moose is an extension of the Perl 5 object system.

Another object system!?!?

Yes, I know there has been an explosion recently of new ways to build
objects in Perl 5, most of them based on inside-out objects, and other
such things. Moose is different because it is not a new object system
for Perl 5, but instead an extension of the existing object system.

Moose is built on top of Class::MOP, which is a metaclass system for
Perl 5. This means that Moose not only makes building normal Perl 5
objects better, but it also provides the power of metaclass programming.