read SOCK, $len_raw, 4;
$len = unpack 'N', $len_raw;
read SOCK, $msg, $len;

and

$len = length $msg;
$len_raw = pack 'N', $len;
write SOCK, $len_raw, 4;
write SOCK, $msg, $len;

or

write SOCK, "$len_raw$msg", $len+4;