All languages that have a concept of standard input/output and environment variables can be used for CGI. The webserver will set the appropriate environment variables (CONTENT_LENGTH, QUERY_STRING, etc.), and pass any POST data on to the program as standard input. And, as with perl, any data printed to standard output is sent to the browser.

If you want to check out some source for a CGI written in C, check this page. The util.c file at the bottom of the page is a commonly-used utility to take care of the parsing of query strings. I believe there are security holes in it somewhere, so I wouldn't recommend actually using it, but it's a good thing to look at to understand CGI in other languages. It's also a good thing to look at to see why perl's string parsing is so well suited to CGI.