perlrequick, perlretut and perlre which usually comes in the perl-doc package are enough to get started.

A really good resource is <http://www.regular-expressions.info/>. It is not perl-specific(C# and Java both have regex, along with most (all?) popular languages)

O'Reilly has 3 books, which you might find helpful :

Regular Expression Pocket Reference, 2nd Edition<http://oreilly.com/catalog/9780596514273/>

Mastering Regular Expressions, 3rd Edition <http://oreilly.com/catalog/9780596528126/>

Regular Expressions Cookbook, First Edition<http://oreilly.com/catalog/9780596520694/>

But mostly, one 'learns' regex by trial-n-error. After many years I still have to test out all but the simplest ones.