The WWW Common Gateway Interface Version 1.1
http://cgi-spec.golux.com/draft-coar-cgi-v11-03-clean.html
を読んでみると、

1.4. Terminology
　　metavariable
　　　　A named parameter that carries information from the server to the script. It is not necessarily a variable in the operating-system's environment, although that is the most common implementation.

6.2. Request Message-Bodies
　　As there may be a data entity attached to the request, there MUST be a system defined method for the script to read these data. Unless defined otherwise, this will be via the 'standard input' file descriptor.

ということなので、CGI1.1的には別に、環境変数じゃなくても、標準入力じゃなくても、いいみたい。