I apologize - I have only read bits and pieces on Mason and I remember reading where it describes itself as a CMS. (No, I don't have the citation. Sorry.)

A long time ago the most interesting free Mason app was "the Mason CMS", which was used at CMP media, where Mason was originally written, IIRC.

You write a Mason app, not a Perl webapp.

Sort of. When I write a "Mason app", most of the logic is still in Perl modules. Mason is really just the frontend. It's just that app server bits it has built in make writing these frontends really simple.