Yes, CHICKEN Scheme. Although I imagine Gambit would work equally well. I have ~3/4 of a single-file CGI framework on my hard drive. It's feature complete: the other 1/4 is just some optimizations and sugar over GET and POST handling, as they are by far the most common methods used.
In the future, I might add some wrappers over access of CGI environment vars, but you can already look those up by hand, using the standard environment access routines.