Вместо test.shtml?parm=N выдаем результат работы /cgi-bin/test.cgi?param=1

    RewriteCond %{REQUEST_URI} /test.shtml
    RewriteCond %{QUERY_STRING} param=[0-9]+
    RewriteRule ^(.+) $1/%{QUERY_STRING}  [C]
    RewriteRule ^/test.shtml/param=([0-9]+) /home/user/cgi-bin/test.cgi?param=$1 [T=application/x-httpd-cgi,L]