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

       ##-- reverse octet order
       reverse

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

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