So, what code have you already written? How does it fail to meet your requirements? Where do you encounter problems?

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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#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1148205

use strict;
use warnings;

while(<DATA>)
  {
  my $pw = reverse map { split //, 999 - $_ } /\.(\d+)/g;
  print $pw;
  }

# want
# 199699309
# 199478688
__DATA__
10.96.3.8
10.113.125.8
[download]