I use perl for debugging the company's products and debugging my personal code.

warnings => enough said.
Data::Dumper => for debugging my code.
LWP::UserAgent => for getting those web pages I have to brea.. errr QA.
HTML::Form => get a list of inputs of a webpage delete /add/modify them. A great way to make the web developer/coder not want to see you. 8)
URI::Escape => Mmm escaped characters made easy for fun and profit.
HTTP::Cookies => cookies made easy and storeable.
Net::Telnet => I used this for some networking research I did at NU.
OpenCA::OpenSSL => This modual saved me a bunch of time and santiy. It drives OpenSSL and the developers have been keeping up the modual almost does everything that OpenSSL can do.