from the CGI manual...

 An attempt to send a POST larger than $POST_MAX bytes will cause para
+m() to return an empty CGI parameter list.  You can test for this eve
+nt by checking cgi_error()...
<snip>
...If the POST was intercepted, then cgi_error() will return the messa
+ge "413 POST too large".
[download]

Cheese :),Ben.