I was aware of Test::MockObject, but not MockModule. However, these are tools for writing tests; I intend Module::Stub to be more general purpose, and useable in production code.
At the moment Test::MockModule is pretty generic despite the namespace. It doesn't include any test functions right now. And if it did, they'd be optional so you could still use it in production code.
See also Sub::Override, which is very similar to T::MM. The main differences are that T::MM allows you override subs that didn't previously exist, and inherited methods. And Sub::Override works on multiple packages, whereas T::MM only operates on one package per object