That seems like a lot of punctuation:

my ($prefix) = join('.',@{[split(/\./,$ipaddr)]}[0,1]);
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Can also be written like this:

my $prefix = join '.', (split /\./, $ipaddr)[0,1];
[download]

Update: Oops.... much like the other "dup" in this thread this was wholly unintentional, and not blatent copying.

-Blake