At the risk of answering questions you haven't actually asked, I've had some success maintaining other people's code using:

• Perl::Critic for hints what they're doing wrong.
• Perl::Tidy to clean up badly formatted code.
• Log::Log4perl to figure out what's called where.
• Devel::Cover to see if your tests cover all the code.

You're certainly right to start writing tests so you won't break anything as you refactor, though. Good luck!