Having a had a quick look, the perltidy prog/site looks like a good place to start.
Properly formatted code is a lot easier to understand.
As mentioned above, the flexibility of Perl can make your task tricky.
If it's fairly straightforward code, then grepping for 'sub ' (ie with trailing space) to find the start of subroutines will help, as you can then use the results to look for sub invocations.
If you also want to lok for the more obvious flaws, (if you have time/have been asked to do so eg you will be maintainer), then using
perl -wc yourfile.pl
and inserting
use strict;
at the beginning of each prog will warn of a lot of potential run-time problems.
Good luck
Chris
PS Perl Medic book idea sounds good, but I've never read it. I can recommend the usual suspects ie Programming Perl (aka Camel), Perl Cookbook. Possibly even Learning Perl (aka Llama book) if you are very new to Perl. All 3 books pub by O'Reilly.