#!/usr/bin/perl
use warnings;
use strict;

my $file = 'C:/Windows/System32/drivers/etc/hosts';
my %replace = ( '#' => q(), '' => '#' );

open my $In,  '<', $file or die $!;
open my $Out, '>', "$file.new" or die $!;
while (<$In>) {
    s/(#?)(10\.10\.1\.2)/$replace{$1}$2/;
    print {$Out} $_;
}
close $Out or die $!;
rename "$file.new", $file or die $!;