in reply to Matching Sequential IP Addresses

Assuming that you want to ensure the hash contains all the IPs with the first 3 octets + 0..255: 

die "IPs missing" unless keys %hashIPs == 256;
my $base = each( %hashIPs) =~ s[(\.\d+$)][];
exists $hashIPs{ $base . $_ } or die "Missing IP $base.$_" for 0 .. 25
+5;

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That assumes that there are no strays in the hash, but it's not clear from your description what it might contain.

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