I've over 30 Perl books, including all the books mentioned in this thread, many of them in several editions. I find their combined total can't hold a candle towards Advanced Programming in the Unix Environment
by W. Richard Stevens - the only books (first and second edition) I've consulted in the past 10 years to assist my Perl programming. And if you want a trilogy, throw in TCP/IP Illustrated
(3 volumes) and UNIX Network Programming
In fact, if I were to choose between the books of W. Richard Stevens vs. all the O'Reilly books (not just the Perl ones), I'd pick the W. Richard Stevens books.