I'd call the differences drastic. HTML::Template is close to a pure view layer. It has very few features and is meant only to display data handed to it. Template::Toolkit is a deep templating mini-language. I have mostly used TT for almost 20 years now and I like it but if you're going to learn something new… it's not what I'd call best practice. HTML::Template is going to be much less confusing and if you decide it's not enough, it will be easy to move up/over. Template::Alloy or probably Text::Xslate are places I'd look first once you're ready (or Mojo’s built-in stuff).

Update… I didn't see this was over a decade old or I wouldn't have replied… I usually only browse by recent nodes. Maybe it got bumped?