
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.
While Modern Perl and Perl Best Practices seem to be now the defacto texts to follow in this field, I highly suggest to also supplement your learning of good Perl by reading Perl Medic by Peter Scott. While its main purpose is how to maintain a legacy codebase, its an absolute gem when it comes to what type of Perl code you should be churning out.
