Since the OP didn't describe the error behaviour properly, here is my attempt:

The server starts OK, and as soon as I start a client the server exists silently.

The client accepts input from STDIN and terminates silently after the second line of input.

Update: I found one error: the server write to $so once instead of writing to $client twice. Fixing that fixes the program.

Second update: please note that this won't work with multiple clients because in the server multiple threads try to read from STDIN at the same time.