A regex here is overkill (since it would end up requiring backtracking at the "mystery" part):

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

I'd much rather use substr() and unpack():

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

No backtracking. Just simplicity.

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;