But in case someone else needs it, here it is:

#!/usr/bin/perl -w
# Get a list of all interfaces and find the local IP and netmask
# Listen for Softros Messenger traffic (UDP port 19771)
# If traffic is received from the local network, do nothing
# If traffic is received from any other interface, resend it to the local
# broadcast address and spoof the original sender IP just as if Linux had
# forwarded the packet
my $localnet = “eth1″;
my $port = 19771;
my ($localnetaddr, $localnetmask, $localbroadcast);

use IO::Socket;
use IO::Interface qw(:flags);
use IO::Select;
# NOTE NOTE NOTE NOTE NOTE This script requires a modification to
# Net::RawIP – the stock version does not set SO_BROADCAST so sending
# packets to a broadcast address results in -EACCESS.
#
# Add this to util.c’s rawsock() just below the IP_HDRINCL code:
# if (setsockopt(fd, SOL_SOCKET, SO_BROADCAST, &val, sizeof(val)) new( Proto=>’udp’,
LocalAddr=>’0.0.0.0′,
LocalPort=>$port,
ReuseAddr=>1,
Broadcast=>1
) || die “Can’t bind: $@\n”;

my @interfaces = $socket->if_list;
for my $if (@interfaces) {
if ($if eq $localnet) {
$localnetaddr = ip2long($socket->if_addr($if));
$localnetmask = ip2long($socket->if_netmask($if));
$localbroadcast = $socket->if_broadcast($if);
}
}

while ($socket->recv($buf, 10000)) {
my ($peerport,$peeraddr) = sockaddr_in($socket->peername);
my $peerip = inet_ntoa($peeraddr);
# Determine if it came in on the local interface or not
if ((ip2long($peerip) & $localnetmask) == ($localnetaddr & $localnetmask)) {
#print “$peerip (local) said $buf\n”;
} else {
# Rebroadcast locally
#print “$peerip:$peerport (remote) said $buf\n”;
#print “resending from $peerip:$peerport to $localbroadcast:$port\n”;
$sendsocket = Net::RawIP->new({
ip => {
saddr => $peerip,
daddr => $localbroadcast,
},
udp => {
source => $peerport,
dest => $port,
data => $buf,
},
}) || die “Can’t create raw socket: $@\n”;
$sendsocket->send;
$socket->recv($buf, 10000); # Immediately eat any data we just sent out
}
}

# From http://weblogs.litmusgreen.com/ed/archives/002250.html
sub ip2long {
return unpack(“l*”, pack(“l*”, unpack(“N*”, inet_aton(shift))));
}