I can't verify this right now, but the theoryx site does have CGI 3.20 listed. My guess is that your original perl installation comes with CGI 3.15, and has it in perl/lib, while the theoryx version gets installed in perl/site/lib. I assume then that the site/lib comes later in your @INC, so that the newer version isn't picked up.

You might try forcing the use of 3.20 by running perl -e "use CGI 3.20; print $CGI::VERSION" or similar. You'd have to use CGI 3.20; in your code as well, of course.