In fact, I think that's nothing to do with RFC. A CGI script can be in Perl, Python, PHP, or whatever, it's the spec only, which is a "WHAT" but I am asking "HOW".
Consider I have already prepare everything (is needed for me at this moment, actually), but as said in my OP, the limitation is I have to do all the rest within my server's code. My server can communicate with the browser by all means.
I don't know "the how" is how to pass the parameters that I've prepared to my target CGI script, so when my CGI script got triggered, it's just naturally have all the %ENV variables ready or a opened STDIN is ready for read().