If you have access to Perlmonks, you have access to a manual. perlman:perl. All the manpages are here.

Also, you might want to check out Martin Fowler's excellent book on dealing with legacy code, Refactoring. Basically, you can't fix code until you understand it, and some legacy code is not comprehensible as written. Refactoring allows you to gradually improve existing code in small steps, checking along the way to make sure you don't break existing functionality.

stephen