One that I use a lot (and encourage others to use) is Perl::Critic. For those for aren't aware, it is a static source code analyzer. It critiques your code against best practices and recommendations from both the Perl community and Damien Conway's excellent book Perl Best Practices.

<rant>A common criticism of Perl::Critic I've heard before is that some people disagree with this or that default policy. So for those folks I recommend Perl::Critic::Lax, which has policies that get Perl::Critic to loosen its tie a bit . There are also 167 modules in the Perl::Critic namespace, many of which are collections of policies and 65 in the Perl::Critic::Policy sub-namespace itself. Chances are that there's a policy in there that might scratch your itch. Failing that they can always RTFM and learn to make their own policies.</rant>

I have found static source code analysis to be a great tool when beginning work on a very large codebase. It helps point out things that could very well be long-standing bugs of which the team working on the code may not even be aware. It also helps me zero in on areas of the code that may have only been put through perfunctory testing that may be in need of extra attention. I highly recommend trying it out if you've never used it.