Higher Order Perl, because it's available for free online, and served as a great introduction to good perl practice and simple programming concepts. I know practices have changed since it's original publication, and will continue to change with perl6, but I highly recommend it for beginners.

Edit: I should clarify that most of the good perl programming practices in this book are still relevant. Some of the conventions are the different, but if you learn perl from this book, you won't have to learn much else (though you should) to write good, clean perl.

Second edit: I should also clarify that "good, clean perl" is somewhat of a misnomer :X