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    my %seen; # Remember IPs we have already seen

...
        next unless /($RE{net}{IPv4})/;
        print IP_DATA "$1\n" if not $seen{$1}++;
    }

    127.0.0.1 host_a
    whoops, no IP on this line
    127.0.1.1 host_b
    127.0.0.1 host_a (duplicate!)
    junk 127.0.0.2 host_c

127.0.0.1
127.0.1.1
127.0.0.2