IMHO the best Perl book is perldoc. ;)
If you must buy a book, I'd say it should be some kind of reference manual, that can be easily referred to while actually writing Perl scripts; the more succinct, the better. My (personal) experience is that thick, verbose introductory tomes on whatever programming language are not very useful - something I rather wish I'd learned
before spending my money on them. My copy of K&R C sees more use as a reference manual for instance than any of my texts on Perl or shell scripting, even though I barely ever use C.
OTOH, if you're doing XML/HTML anything then you're probably a better programmer than I am, so take the above with a grain of salt...
Edit: sorry, didn't realize this
thread was a month old. D'oh!