use Socket;
require "sys/socket.ph";    # for &TCP_NODELAY

setsockopt(SERVER, SOL_SOCKET, &TCP_NODELAY, 1)
    or die "Couldn't disable Nagle's algorithm: $!\n";

Re-enable it with:

setsockopt(SERVER, SOL_SOCKET, &TCP_NODELAY, 0)
    or die "Couldn't enable Nagle's algorithm: $!\n";