You're getting that error because your script isn't outputting any HTTP headers. Try this FAQ: Where can I learn about CGI or Web programming in Perl?

Update: The link I was looking for on that faq page is broken, so here's another one: CGI Programming on the World Wide Web. This is the online version of the O'Reilly book I learned CGI from. It's a bit dated (out of print in fact), so you should consider picking up CGI Programming with Perl, 2nd Edition from your local book store.

-Matt