Only three? Aw, gee, that's what I have in my standard perl script/module template! strict, warnings, and diagnostics - and this is just to keep me from doing stupid things, nevermind actually accomplishing work.

Now, assuming that you're going to allow all the pragma modules we want, I've found the most use so far from CGI::Application (even though there are aspects of its design that still bother me, I just haven't had time to go in and take what I like about it and put it in a module that has nothing I dislike), XML::Twig (syntax is a bit heavy, but that also gives me a lot of flexibility), and HTML::Template (the nice thing about HTML::Template is that it is a complex piece of machinery hidden behind a relatively simple API - no need for anyone to write HTML::Template::Simple!).

Of course, the other issue with the question is that I need to write domain-specific modules, and the way the question is phrased, it sounds like that isn't allowed. I'm going with the spirit of the question with the above three modules, not the letter of the question. :-)

With the spirit of the question, though, I have to say that there are a lot of other simpler modules (smaller API and smaller scope) which I find useful as well which may be a more reasonable place to start. Text::Wrap is just too cool - and is standard as well. Not difficult stuff, but at least I don't need to think about edge cases ;-) Term::ANSIColor (even though they misspell 'colour' ;->) is a nice, readable way to make things stand out in my output. LWP::Simple allows you to easily download webpages (or any other web resource). Those would be my "simpler" 3 pick.