Mason allows you to embed small perl fragments in your HTML documents. It also provides a rich component system with the concept of inheritance. Since it mixes Perl and HTML, it is easy to abuse Mason and associate interface code with application logic. Mason is a very capable library, but it is not a web framework. It takes an intermediate place between the scope and the library of templates. There is a book about using Mason available on the Internet.
Catalyst is a popular web application framework . In other words, it provides the infrastructure for interacting with the server, communicating with users, authentication and data access. The standard template library used with Catalyst, TemplateToolkit . However, you can use Mason as your template system with Catalyst.
You can use these two libraries together or separately.