Definition:
CPAN stands for the Comprehensive Perl Archive Network, which lives at http://cpan.org.
CPAN is a repository of freely-redistributable code you can use in your projects. Its philosophy is summarized quite nicely by its strapline, which is: "Stop reinventing wheels, start building space rockets!"
So, for instance, if your script needs to do something specific, such as interface with a web service which is returning its results in JSON (JavaScript Object Notation), you can hop across to CPAN Search and see if anyone else has written some Perl which can parse JSON responses (they have!)
You can share the fruits of your labors with the Perl community at CPAN too. Find out more about submitting modules at the CPAN FAQ.