Is Perl/CGI the best language to use for sending a customer to a thank you page after they click the submit button? If they're not, then what is?

I would say so, but I'm biased to Perl/CGI. It ultimately depends on what you are trying to do. When they click submit, what exactly do you want to happen? For some reason, these aren't listed under Tutorials (where they should be) but, nevertheless, I have written a few tutorials covering basic and simple CGI's with Perl, hopefully to let people see it's power....
http://www.daniweb.com/techtalkforums/thread21269.html
http://www.daniweb.com/techtalkforums/thread21279.html
http://www.daniweb.com/techtalkforums/thread21830.html