I second CountZero and perrin. I've started using Template::Alloy lately and I really like it so far. Mason is a solid, widely used, powerful kit. It's also the closest in style to PHP. This is what perrin was warning about, however. PHP style is generally terrible. (Perl can be, and too often is, coded this way too but PHP presents it as the first avenue so that's where most folks drive.)

The moral there being, if you prefer that style, go for it. If you are hacking big stuff however, you will find that at least some near MVC or hybrid of it with concerns separated as cleanly as makes sense gives you a lot more time to have fun and requires a lot less time debugging, shimming in noodles, and cursing the day you thought to yourself, "Oh, I'll just do it this way, it'll be faster since I already know it."

You can use Mason, any view or combination of them at all, with Catalyst too.