As you've read the RFC, maybe you can tell us what parts of chapter 3, Invoking the Script you've already implemented, and where your server still fails.

Update: Chapter 4.1, Request Meta-Variables tells me:

Meta-variables contain data about the request passed from the server to the script, and are accessed by the script in a system-defined manner.

The traditional system-defined manner is using the environment (%ENV) to pass these values. See also chapter 7, which discusses the different operating systems and their system-defined ways of communicating the variables to the child process.

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().

The RFC describes what the server must do that the script finds the information.

The "how" also is evident from the RFC. You stick the informatin into %ENV. Also consider looking at HTTP::Server::Simple::CGI::Environment or HTTP::Request::AsCGI, which set up a CGI environment.

I read the RFC recommended,I know what I should prepare (and I already knew) for my script, but the question is how to send it to the script.

client makes a request through socket , your server reads headers, then

Populate a local %ENV

dispatches/calls/invokes a process ,

through pipe, fork, whatever, see perlipc, Capture::Tiny

put http request BODY on STDIN

Collect output from STDOUT

log errors from STDERR

After timeout, or whatever, return/generate/forward a return HTTP request back on socket to the client

I already linked examples in Re: How a web server sending data to a CGI perl script ?

Thanks and I am still reading.... I can feel something and it's insightful indeed. Would be glad to hear more from your advise though :

1. Do you think rather send out the data to an external script, import the external code into the server and run it is a better approach (eg. require "script.pl") ?

2. Can you tell if when a web server claims it supports a certain language, it does have to implement something ( not just command line ) to prepare the environment ( meta ) variables for that particular language ?

1. Do you think rather send out the data to an external script, import the external code into the server and run it is a better approach (eg. require "script.pl") ?

Sure its a better approach (hint CGI::Compile/ ), for perl, its called PSGI as implemented by Plack

2. Can you tell if when a web server claims it supports a certain language, it does have to implement something ( not just command line ) to prepare the environment ( meta ) variables for that particular language ?

What "web server" are claiming what kind of support?

Yeah, question is kinda vague, claimed support can be anything, from what mod_perl does (full control) to what to merely support for better-than-CGI like WSGI, PSGI, Rack, JSGI, SCGI...