UDP does not work in a “client/server” manner, it works in a peer-to-peer manner, this piece of demo might help you to understand this:

test1.pl:
use IO::Socket::INET;
my $out = new IO::Socket::INET(Proto => "udp", PeerAddr => "localhost"
+, PeerPort => 3000, Timeout => 10) || die "failed";
my $in = new IO::Socket::INET(Proto => "udp", LocalAddr => "localhost"
+, LocalPort => 3001, Timeout => 10) || die "failed";
print $out "1\n";
while (1) {
    my $msg = <$in>;
    chomp $msg;
    print "got $msg\n";
    print $out $msg + 1, "\n";
}

test2.pl:
use IO::Socket::INET;
my $in = new IO::Socket::INET(Proto => "udp", LocalAddr => "localhost"
+, LocalPort => 3000, Timeout => 10) || die "failed";
my $out = new IO::Socket::INET(Proto => "udp", PeerAddr => "localhost"
+, PeerPort => 3001, Timeout => 10) || die "failed";
while (1) {
    my $msg = <$in>;
    chomp $msg;
    print "got $msg\n";
    print $out $msg + 1, "\n";
    sleep 1;
}
[download]

Or use one socket for both in and out, and do it in this way:

test1.pl:
use IO::Socket::INET;
my $socket = new IO::Socket::INET(Proto => "udp", 
                               LocalAddr => "localhost",
                               LocalPort => 3000,
                               PeerAddr => "localhost", 
                               PeerPort => 3001, 
                               Timeout => 10) 
        || die "failed";
while (1) {
    my $msg = <$socket>;
    chomp $msg;
    print "got $msg\n";
    print $socket $msg + 1, "\n";
}

test2.pl:
use IO::Socket::INET;
my $socket = new IO::Socket::INET(Proto => "udp", 
                               LocalAddr => "localhost",
                               LocalPort => 3001,
                               PeerAddr => "localhost", 
                               PeerPort => 3000, 
                               Timeout => 10) 
        || die "failed";
print $socket "1\n";
while (1) {
    my $msg = <$socket>;
    chomp $msg;
    print "got $msg\n";
    print $socket $msg + 1, "\n";
}
[download]

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