use XML::XPath;

my $xp = XML::XPath->new(filename => '/path/to/file');

my %seen;
foreach ('IP-ADDRESS', 'IP-NEIGHBOUR') {
  foreach my $ip ($xp->findnodes("//$_")) {
    my $addr = $ip->findvalue('.');
    print "$addr\n" unless $seen{$addr}++;
  }
}