Looks like the last time this was asked, the norm was a bit lower.

1. Perl 5 Pocket Reference
2. Data Munging with Perl (*2)
3. Official Guide to Programming with CGI.pm
4. Learning Perl/Tk
5. The Perl CD Bookshelf
6. Perl in a Nutshell
7. Perl Cookbook
8. Writing Apache Modules with Perl and C
9. CGI Programming with Perl
10. Perl for System Administration
11. Mastering Algorithms with Perl
12. Advanced Perl Programming
13. Programming the Perl DBI
14. Effective Perl Programming
15. Programming Perl (3rd)
16. Programming Perl (2nd)
17. Programming Perl (1st)
18. Object Oriented Perl

(19 books, 14 distinct titles)

The books from this list I've read cover to cover would be a much shorter list.