in reply to Optimizing existing Perl code (in practise)
The first thing would be to find where your program spends the most time, and look to see if that can be improved. You might look at the Devel::* modules, such as Devel::DProf, to help with this portion of the process.
You might also wish to consult several of the excellent books available. Some of the books I am aware of (and own) that might be helpful include Programming Perl,
Advanced Perl Programming,
Mastering Algorithms with Perl, and Perl Cookbook.