Hands down this is probably the single best library on CPAN.
I started web development with the attitude of "I'll just roll my own template system with regexes" and "why create another mini-programming language when you already have perl?".
Let me tell you something. I was wrong. My regex's sucked, my home-brew cache code was buggy, and the way the templates were stored (in the database) made version control a nightmare. Template::Toolkit and it's family of plugins have stripped out all the wastefull, trivial, ugly loop crap (i.e. a for() loop for a bunch of <select> options) in my codebase and moved them into very elegant templates. My perl codebase was literally cut in half.
Once you see what a template library like this one can do to your code, you'll wonder how you ever did web applications before. Trust me, you dont know how to roll your own template language. You dont have the time to piss away rolling your own cache code and no matter how hard you fight, the ability to do simple loop or conditionals in your templates *is* the right thing to do.
What does my rambling say? Template Toolkit rocks!
Could have wrote it myself but it beats having to do the math every time I need to do proportional sizing.
Took a while to figure out both conceptually and technically, but this little puppy saved me a ton of time trying to write my own "glue" for serving images. The source code is great documentation.