See the online chapter from Lincoln Stein's book Network Programming with Perl, about Net::Telnet and Net::FTP: chapter 6.

I must say I'm having a hard time imagining using FTP from inside a CGI script as a good idea. It seems like two separate worlds to me. And note that you can also use LWP (even LWP::Simple) to just download a file over FTP. Often that's a simpler solution. All you need to do is construct and use an ftp://... style URL.