Not a GLOB reference at tcpserver4.pl line 43 (and 83).

Would you try the following changes:

use threads qw[ yield ]; ## Why (*&^^%) isn't that exported by default
+?
...
    threads->create(
       ...
    );
    yield; sleep 1;      ## insert
}
...
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1. "Old threads within the server persists for at least 10 hours"

   I just don't see that effect here. No matter how I break the connection--from quitting the client cleanly to killing the client process--the while( <$client> ) loop in the server terminates immediately, and so the thread ends.

   This seems to be one of those infamous platform differences. On windows, there are timeouts for inter-character receipt, a per-byte-to-read timeout, and a total read timeout.

   Looking at the man page for SetSockOpt(), you could try using SO_RCVTIMEO to set a maximum read timeout and see if that allowed the read to terminate.

2. " it immediately sends the command to the Selected client without delay."

   Hm. It implies that on your platform, writing to a (dup of) a socket, whilst another thread is in a read state on it, goes ahead immediately.

   That doesn't work on here. (At least for sockets as provided by IO::Socket::INET--it seem to work okay from C.)

3. "I do include with client command data also the UnitID to whom it must send the command."

   I've been doing that too. My code (when it works:(), routes the commands using the UnitID input from the command client.