What benefits would using the mixin module have? It doesn't look very different from simple Exporter approach. If it has a benfit, I would think it's that you are declaring "Hey! A mix-in class is being used here!". And maybe that's enough benfit to make it worthwhile.

In particular, could anyone who used the mixin module comment?

I maintain a mix-in module myself, using the Exporter method: CGI::Application::ValidateRM.