Just as others, not exactly sure what you want. Assuming all you want is that something called "/index.html" to actually call "/index.cgi" (I'm guessing may be your CMS is explicitly linked to index.html?), it's rather hard to do in standard cgi, but if you use mod_perl, then it's pretty easy to add something like:

<LocationMatch "index.html">
   SetHandler perl-script
   PerlHandler YourIndexModule
</LocationMatch>
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