It seems I can use 2 ways below to treat it:

Looking at the protocol, (which is pretty badly designed IMO), you're going to have to use a mixture of both methods.

1. First you need to read 1 byte to determine if it is a fixed (0x10) or variable (0x68) length packet.
2. If the former, you can then read the next 5 bytes, check the checksum and unpack the payload.
3. If the latter, then you need to read the next two bytes to obtain the length.
4. Then you add 3 to that. (To account for the second start byte, the checksum and stop bytes.)
5. Then you can read the rest of the packet, check the sum and unpack the payload.
6. And finally ack (0xe6) or nack (0xa2) the packet.

Don't forget to binmode your socket and use sysread. I've found that wrapping sysread in a sub that checks the return code and returns the result for assignment makes life much easier.

That still leaves the problem of timeouts. There are various ways of tackling that--alarm, ioctl, select etc. Which is appropriate will depend a lot on you platform. That said, tcpip comms is pretty reliable especially if LAN connected. It may be a sufficiently infrequent occurrence that you can essentially ignore it. That will depend upon your circumstances and network.