OP asks "How CGI becomes on-topic.It is "Common Gateway Interface" isn't it."

ahh.... <sigh>.....Indeed it is, and that definition does not rule out C or any other language. But... to be on topic here, your question should have a specific perl component; in other words: we'll be most receptive if your question relates to using perl with the CGI protocol

    Well, in reality, I think you'll find most questions entitled or labeled with CGI here will draw a common reaction/assumption/inference -- namely, that "OP is asking about using a perl script, in a CGI context." unless the question specifically asks about some other language.

OTH, many questions re Apache shade towards "how do I make Apache do x?" That's a question best directed to the folks who wrote/maintain/focus on Apache. MySQL questions directed to interfacing with perl tend to be received, receptively... but not the questions that boil down to "I haven't bothered to study the documents, but please teach me MySQL." Same goes for .html questions: you may find Monks forthcoming about how to template a web page and use perl to keep it up to date -- since their are modules specifically for that purpose... but when the question runs toward "what's wrong with a <p> tag inside a <span style="futz:foo; bar: blivitz;"> ?" -- well that's better directed to the readily available documentation (see, for example, http://www.w3c.org) or communities where .html is "Topic A."

And if you don't know what's an appropriate site to post some particular question, you probably haven't taken the trouble to learn enough of the basics of the subject to guide you. And that does tend to make folks grumpy.