The first place to start is perldoc perlre. If you're in a Unixy environment, just type that at a command prompt (I think it works that way in Windows too, but I rarely program there). With the Activestate compiler, HTML documentation is installed in your Start menu.

From there, two excellent books are Programming Perl (from O'Reilly) and Mastering Regular Expressions (also from O'Reilly). Both are a little bit dated, but they'll have 98% of the information you're likely to need. Also check out places like perl.com for more up-to-the-minute information.