What do you have to do to make your symptoms appear using this script?

Okay. I'll play :)

1. Start your server in one session.
2. Run this in a second session:

   perl -MIO::Socket -E ' $s=IO::Socket::INET->new("localhost:1200"); $s-
   +>send( "a" ); sleep 1e6 '
   [download]

3. Now start a third session and telnet into your server.

   You'll get a connection and be able to type stuff; but you'll get no replies and none of your telnet input will be displayed on your server's console.

4. Start as many more clients -- telnet or otherwise -- as you like.

   Your server will never see anything from any of them until you kill that perl one-liner. (Or you wait the 11.57 hours DAYS for the sleep to time out.)

The reason is that the send('a') caused select to return a readable file handle. Your server then attempted a readline from that client; but the client never sends a newline, so the readline never returns and your server is dead in the water. The tcpip stack will service connections, but your server will never loop back to accept them.

One bad client and your server is DOS'd. This is the exact scenario that I described above with that "teaching material"; the source of my "obsession with readline".

You know I'm sure, that multiplexing with IO::Select is only meant for use with short messages; if you are doing large data transfers, you need to use fork or threads.

Absolutely not!

If you use recv (or sysread), thus avoiding line-based and buffered IO, you can service huge data packets and small ones; you simply accumulate partial packets in buffers and only process input once you've accumulated enough to satisfy the comms protocol requirements. Whether that is newline (or other character sequence) terminated records or length pre-fixed; or any other mechanism.

(Oh. And don't forget to set the sockets non-blocking!)