You're close; the next step is to keep track of the IP addresses you have already seen, and print to your output file only if you encounter a new one. I would structure your loop like this:

    my %seen; # Remember IPs we have already seen

    while (<NMAP_DATA>) {
        next unless /($RE{net}{IPv4})/;
        print IP_DATA "$1\n" if not $seen{$1}++;
    }
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I used the following test data:

    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
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And got this output:

127.0.0.1
127.0.1.1
127.0.0.2
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