Two great places for more information on Mason:
Embedding Perl in HTML with Mason
by Dave Rolsky & Ken Williams (full text online too!)
Mason HQ for all your fully up-to-date mason info and discussion.
From the preface to the mason book:
"Mason is a tool for embedding the Perl programming language into text, in order to create text dynamically, most often in HTML. But Mason does not simply stop at HTML. It can just as easily create XML, WML, POD, configuration files, or the complete works of Shakespeare."