I have three additions:

• Best sub-skill Perl-related book: Mastering Regular Expressions: Powerful Techniques for Perl and Other Tools
  Perl-centric, but focused on providing an overall understanding of regular expressions. Reading this book has already revolutionized the way I approach data filtering and transformation problems, and I'm not even entirely finished with it. Great reference guide, as well.
• Best Perl reference book: Perl Cookbook
  Not only solid solutions for common challenges, but a discussion of the various approaches and their merits or drawbacks.
• Best Perl book on SOAP and related tech: Programming Web Services with Perl

Yeah, the last is specialized, but a fabulous book. Interestingly enough, all three books I listed were O'Reilly publications, so it bears saying that I have no stake in O'Reilly -- just a happy customer. ;-)