On the chance that you phrased your question a bit too narrowly, I'll answer the broader question "Is it possible to invoke a CGI script instead of index.html?"

Yes. Details for how to do this depend on what web server you're using. Apache and IIS can both be configured with a list of names to try to figure out how to handle   http://www.example.com/ index.cgi can be on that list. Assuming you've also configured the web server to execute CGIs, index.cgi will get invoked no other file on the list is found first.

With Apache, you can arrange, on a per-directly basis, to have .html mean CGI. Consult the Apache docs.