The receiver is going to need to know when it has received a whole message. The following does so by prefixing the data with the length of the data.

Sender:

my $msg = pack('N/a', nfreeze($data));
while (length($msg)) {
   send_msg_block(substr($msg, 0, $max_msg_size, ''));
}
[download]

Receiver:

while (length($buf) < 4) {
   read_more_into_buf($buf);
}
my $data_len = unpack('N', substr($buf, 0, 4, ''));
while (length($buf) < $data_len) {
   read_more_into_buf($buf);
}
$data = thaw(substr($buf, 0, $data_len, ''));
[download]

This doesn't address the issue the anonymous monk raised about UDP. That's for your send_msg_block and read_more_into_buf to handle.