I have a feeling that ETX in this case stands for the ASCII character Control-C (ASCII code 3). Similarly, STX stands for ASCII code 2, hence the reference to chr(02) and chr(03) in the OP's code. Perhaps the OP can verify that this is the case. In any case, setting the input record separator for the socket should work, e.g.:

$sock->input_record_separator(chr(3));
while (<$sock>) {
  ... # $_ contains one complete message
}
[download]

For some history on what codes like STX and ETX originally were used for, see The ASCII Control Characters.

Update: Just noticed this suspect line in the server code:

$STX=chr(03);
[download]