You can set up a callback that will be called whenever a file upload is being read during the
       form processing. This is much like the UPLOAD_HOOK facility available in Apache::Request, with
       the exception that the first argument to the callback is an Apache::Upload object, here it's the
       remote filename.

        $q = CGI->new(\&hook,$data);

        sub hook
        {
               my ($filename, $buffer, $bytes_read, $data) = @_;
               print  "Read $bytes_read bytes of $filename\n";
        }