I've been programming with Perl for quite sometime now and find myself constantly refering less experienced Perl programmers to make more use of the core Perl modules. For example, I've had to explain the use of strftime more times in the past month than I can remember.
Despite this, I rarely stray far from the core modules other than to use the various DBI/DBD, XML, and LWP modules. I just haven't needed to step to far from these modules, or so I seem to think. What modules outside the core are essential for you to get your work done an why? Is there some necessary module that you think I've missed?