You don't have to intermix HTML and Perl -- you just can when you need/want to.

You can also separate your application into some Mason documents that present data and others (or better yet, non-Mason Perl Modules) that hold the business logic and interact with your database.

Mason provides syntactic sugar to control the interface between the two and by allowing perl in your presentation layer, you have a great deal of power/flexibility in generating HTML.

Mason also allows nice modularity of various presentation elements and combining them in various ways to produce user interfaces.