[Conway 99] Conway, Damian. Object-Oriented Perl. An excellent book describing object-oriented programming using Perl, from the basics to advanced topicsa must-read!
[Stein+ 99] Stein, Lincoln, and Doug MacEachern. Writing Apache Modules with Perl and C. All the details of writing mod_perl programs can be found in this book.
[Wall+ 00] Wall, Larry, Jon Orwant, and Tom Christiansen. Programming Perl, Third Edition.