Not the cleanest but it works. BTW you can't also separate on 0, all you care about is the unique octect that does not == 255. Your logic was a little off.

My route: I dumped the entire IP address array inside the subnet array for simplicity. Then I iterated over the subnet array and did the calculations and 0 assignments.

sub getmask
{
   push(@sub, @ip);

   my $cnt = -1;
   foreach my $line (@mask)
   {
     $cnt++;

     if ($line != 255)
     {
         if ($flag > 0) { $sub[$cnt] = 0; }

         my $magic = 256 - $line; 

         my $rem   = $ip[$cnt] / $magic;
         $rem =~ s/\.(.+)//;

         $sub[$cnt] = $magic * $rem;

     }
   }
print join(".", @sub);
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