Regular Expression Cookbook
This is a fantastic collection of mini tutorials and code snippets using regular expressions.
A Tao of Regular Expressions
A nice tutorial on working with and thinking about regular expressions from Steve Mansour.
Regular Expression Library
Although these are built mostly in ASP, many of them work or are easily translated to Perl. It's a nice resource.
Simple Matching With Regular Expressions
A gentle introduction to Perl's regular expressions. In this tutorial we look at simple string pattern matching.