This raises a good point: you have a number of tcp and udp sockets. Do you need (or want) to operate them at the same time (i.e. multiplex the I/O)? If so, you can either do that with threads or also with select.

Final point... select can also be used to determine if a socket is ready to accept an incoming connection.

Thanks folks. Both the non-blocking and select responses helped. I've got working code now; I ended up not needing to establish non-blocking, as pc88mxer's guidance put me in the right place. Here's my final working routine:

Sorry that this looks ugly here. click download to see correct format.

sub ListenOnUDP
{
    my $end = time + $timeout;
    print "PortListen on $strFQDN ($address) at $strProtocol\/$intList
+enPort\n";
    # Open a udp port
    my $hSocket = IO::Socket::INET->new(LocalPort=>$intListenPort, Pro
+to=>$strProtocol, Timeout=>$timeout)
    or die "Can't create UDP socket: $@";
    
    # Wait for a connection ($hSocket is undef until an accept call is
+ made).
    my $rin = '';
    vec($rin, fileno($hSocket), 1) = 1;
    while (1) {
        my $timeleft = $end - time;
        if ($timeleft <= 0) {
            print "No connection: Timed out after $timeout seconds.\n"
+;
            last;
        }
        my ($nfound, $t) = select(my $rout = $rin, undef, undef, $time
+left);
        if ($nfound == 0) { # either timeout or end of file
            print "No connection: Timed out after $timeout seconds.\n"
+;
            last;
        }
        
        # Connection exists; Receive data, open a response port, respo
+nd, then close both ports.
        my ($datagram,$flags);
        while ($hSocket->recv($datagram,42,$flags)) {

            my $ripaddr = $hSocket->peerhost;
            my $rport = $hSocket->peerport;
            print "RECIEVE_FROM-TO, $ripaddr, $strProtocol\/$rport, $a
+ddress, $strProtocol\/$intListenPort, message=$datagram\n";

            # UDP is connectionless, so try to open a connection going
+ back the other way, then send a response.
            # NOTE: This requires a predefined 'remote' port. Pass via
+ commandline; default is $intSendPort
            my $hResponseSocket = IO::Socket::INET->new(Proto=>$strPro
+tocol,PeerHost=>$ripaddr,PeerPort=>$intSendPort);
            print "SEND_FROM-TO, $address, $strProtocol\/" . $hRespons
+eSocket->sockport . ", $ripaddr, $strProtocol\/$intSendPort, message=
+PONG!\n";
            $hResponseSocket->send("PONG!");
            close($hResponseSocket);
            last;
        }
        last;
    }
    close($hSocket);
}
[download]

Again, thanks folks, this solved my problem. Anyone interested in helping me QA and test my final script on other platforms?