($flag1, $bytecount, $mystery, $flag2) = $frame =~ m{
  ^
  ( [01]{8} )
  ( [01]{16} )
  ( [01]*? )    # that could be [01]* for all I care...
  ( [01]{8} )
  $
}x;

##</code><code>##

($flag1, $bytecount, $mystery) = unpack "A8 A16 A*", $frame;
$flag2 = substr($mystery, -8, 8, '');