{   
   short lengths[2];
   char data[???];
  }
where there are two items in data,  of length
lengths[0] and lengths[1] respectively.

##</code><code>##

eval('($junk1 , $junk2, $data0, $data1) = unpack "ssA' . $data0_length . "A" . $data1_length . '",$rcvd_buf;');