I would also encourage you to read the relevant manpages. http://learn.perl.org/ is also a good resource. Assuming you just want a quick fix, this might do the trick:

$out = 
  ##-- concatenate result digits
  join('',
       ##-- reverse digit order within each octet
       map {reverse split(//,$_)}

       ##-- reverse octet order
       reverse

       ##-- subtract octet values from 999
       map {999-$_}

       ##-- extract final 3 octets
       ($ip =~ /^\d+\.(\d+)\.(\d+).(\d+)$/)
      );
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