Do you want to do this because the request body isn't a form? If the request body isn't a form (application/x-www-form-urlencoded or multipart/form-data), then CGI provides the request body in $cgi->param('POSTDATA').

If you want to do your own form handling, you could set $ENV{CONTENT_TYPE} to application/octet-stream (or whatever) and fetch the request body using $cgi->param('POSTDATA'), or you could read STDIN and set $ENV{CONTENT_LENGTH} to zero before creating the CGI object.