Math::Category is a Perl module that defines categories and mathematical structures like functors, natural transformations, and monads. It implements categories as objects with morphisms that have source and target objects. Functors map objects and morphisms between categories while natural transformations transform functors. The module also defines monads as endofunctors with additional structure and provides examples like the list and state monads. Kleisli categories are constructed from monads to model computations.