Somehow I did not believe you cannot use a UDP socket to broadcast and receive responses. When I looked I had C code which does precisely this. The problem appears to be my use of IO::Socket or IO::Socket as if the client code above is replaced with the following it all works fine. The PeerAddr causes IO::Socket to do a connect so the only way I can make IO::Socket do it was to remove PeerAddr (and hence PeerPort) and change the send to use INADDR_BROADCAST. 
socket(my $socket, AF_INET, SOCK_DGRAM, getprotobyname('udp'));
setsockopt($socket, SOL_SOCKET, SO_BROADCAST, 1);
my $destpaddr = sockaddr_in(9990, INADDR_BROADCAST);
send($socket, 'Q', 0, $destpaddr);
my $wait = IO::Select->new($socket);

while (my ($found) = $wait->can_read(10)) {
   my $srcpaddr = recv($socket, my $data, 100, 0);
   my ($port, $ipaddr) = sockaddr_in($srcpaddr);
   print "Read " . (gethostbyaddr($ipaddr, AF_INET) || "UNKNOWN") . ",
+ " . inet_ntoa($ipaddr) . ", ", $data, "\n";
    }
    close $socket;

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i.e. you can open a udp socket, broadcast on it and then receive data on it sent from the processes seeing the broadcast.

In reply to Re: How to answer a UDP broadcast by mje
in thread How to answer a UDP broadcast by mje

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