Well, you'd need to set "ultimatebb.cgi" as an index document in your webserver; either through .htaccess or the apache httpd.conf.
The line to edit in the httpd.conf (for the specific site) or the line to add to the .htaccess file is:
DirectoryIndex ultimatebb.cgi index.html index.htm index.php index.php4 index.php5
You could also use an index document to forward the request to the file; there are many ways of doing this; the easiest of which would be to create a file containing the following data with the name "index.php" (though I'm not sure how this would work with including a perl page; you may just do a html redirect):
include("ultimatebb.cgi");