Two excellent books I can recommend for someone working with legacy Perl:

Perl Medic (Peter J. Scott) - http://amzn.com/0201795264 Effective Perl Programming (Joseph N. Hall, Joshua A. McAdams, brian d foy) - http://amzn.com/0321496949

Besides those, having a good reference to the language is also essential. For example, I keep Perl In a Nutshell close by, even though I almost always just search http://perldoc.perl.org/ when I need that info.

I wish I had kept it, but I can't seem to find it in the set of files I kept from that job. It would have been fun to make a blog post out of it.

Googling for "things I hate about Perl" turns up a number of articles -- often written by folks who like Perl 5 but who also are happy to admit to its issues.