Looks like about 21 or so.

From O'Reilly,

1. Advanced Perl Programming,
2. CGI Programmming with Perl,
3. Learning Perl,
4. Learning Perl/Tk (?),
5. Mastering Algorithms with Perl,
6. Mastering Regular Expressions,
7. Perl Cookbook,
8. Perl in a Nutshell,
9. Perl for System Administration,
10. Perl 5 Pocket Reference,
11. Perl/Tk Pocket Reference,
12. Programming Perl,
13. Programming the Perl DBI,
14. Writing Apache Modules with Perl and C, plus
15. Webmaster in a Nutshell.

From other publishers,

1. Building Business Web Sites,
2. The CGI Book,
3. Data Munging with Perl,
4. Instant Web Scripts with CGI/Perl, and
5. Object-Oriented Perl.

I have also misplaced/lost (be it by moving, or losing touch with people) at least 1 or 2 books, including

1. Perl by Example (which I think came out shortly before or about the same time 5.x was released).

I may have others-those are the ones I can recall or have within easy reach at the moment.

My many thanks to those authors, who set my foot upon the path of Perl, and taught me its sign posts.

Thanks also to Jazz, who in Re: How Many Perl Books Do You Own?, proved I still have some to go before I am a full-blown Perl junkie (still just a mild but fun addiction yet).