I'm still new, so as far as CPU volume, I think I've used Perl most for an online budget and stock market server for a Play By Committee strategy game I'm running.

It has a very high niftiness factor, since I was able to write the whole thing myself as a "Learn Perl project" with just a little bit of socket help from a template.

It started out as a telnet program, with regexes to parse instructions like "buy 5000 radioactives at maxprice 250 minerals", and I expanded it into a webserver with colours and icons and buttons :) (works great under anything but certain versions of IE :P)

And yes, I used strict, warnings, taint checking and paranoia right from the start