use strict;
use warnings;

my %seen;
my @sortedUniques =
   map  { ( split m{\s+}, $_->[ 0 ] )[ 0 ] }
   sort { $a->[ 1 ] cmp $b->[ 1 ] }
   grep { ! $seen{ $_->[ 1 ] } ++ }
   map  {
            m{^(\d+)\.(\d+)\.(\d+)\.(\d+)\t}
            ? [ $_, sprintf q{%03d%03d%03d%03d}, $1, $2, $3, $4 ]
            : ()
        }
   <DATA>;

print qq{$_\n} for @sortedUniques;

__END__
# hosts file

10.31.17.65	fw2
192.168.1.78	hosta
192.168.1.21	hostb
10.31.17.65	fw2dup
10.23.212.6	hostc
192.168.100.254	fw1
192.168.1.21	hostbdup