The most classic answer is CGI.pm. This is a module that does most of the overhead of interacting with the web. Perl is perfect for CGI programming because such things are so often text-based (one of perl's strengths), require quick development (ditto), and flexibility (yup).

I'd recommend you look at O'Reilly's CGI Programming with Perl as a starting point. It's pretty basic, but thorough.

(CGI is used for receiving input from users, LWP for sending and requesting pages).

If you need to parse a lot of HTML, check out HTML::Parser, and The Perl Journal article about it in the current issue. (pay subscription) (Article is recommended, because the HTML::Parser documentation assumes you know what it does and how)