Somehow, the Perl Black Book became my most used and indispensable dead-tree reference. I barely remember, but I think it's what I used to learn Perl. I've since outgrown needing it, but it's worn, coffee-stained pages are still within reach at all times.
Whew! I was starting to worry that I might be the only one... I don't use my copy much any more either, but I still find it to be a decent reference when I've either been up too long or away from Perl for too long and can't remember the details of how to do something.
My most-referenced book lately has been a decade-old copy of HTML: The Definitive Guide. I have a long history of sticking to <ul>s and <table>s for all my web formatting needs, but I've recently started doing more stuff where that's not sufficient.
Underneath that in the pile are Perl in a Nutshell, Perl Cookbook, Programming Perl, and the aforementioned Perl Black Book.