The CGI module (or CGI.pm to distinguish it from the CGI protocol) is a very core Perl module that helps write CGI scripts and other CGI-like things. For very simple scripts, CGI.pm is a bit overkill, but it's very helpful and useful for anything substantial.
You can use the module to create Web fill-out forms, do stuff with form content, read/set cookies, muck around with HTTP headers, create HTML... Oh gosh, just read the POD. If you're writing CGI scripts in Perl, use CGI.pm unless you have a good reason not to.
code('Perl') || die;