in addition to what everyone else has pointed out, HTML::Template has the advantage of being relatively language independent. the template syntax isn't tied to perl, and there are implementations written in python, php, java, and probably other languages. so if you ever decide to switch languages for the backend, you don't have to port the templates. i've successfully converted apps from perl to python and vice versa without touching the template files.
i've also used HTML::Template to make 'skinnable', or highly customizable web apps. you just throw up a quick admin interface that lets users (or admin users at least) edit the template files that generate content directly through the browser. i have, eg, a quiz/survey/poll engine that uses HTML::Template. it's designed to plug into any site or web application that we need to plug it into. using templates lets us easily customize each instance to fit in graphically.
and HTML::Template's ability to hook into the CGI and DBI modules, when used properly, reduces a lot of the extra overhead.