Your server is only reading one line from the client. If your client sends multiple lines, it's possible that the server has closed the socket before the client finishes sending all of his information. In this case, the attempt to print to the socket is probably getting a SIGPIPE aborting your client before it even gets a chance to read the socket. By adding the sleep() call, you have given the client more time to finish writing all of the extra data to the socket and it can complete successfully.

Try adding $SIG{PIPE} = sub { print "SIGPIPE\n"; }; to your client code somewhere near the top. If 'SIGPIPE' ever gets printed at the client end, then what I described above is happening.