In my CGI.pm, that passage reads,

           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();
        $q->upload_hook(\&hook,$data);

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