Also, (#)? would be more readable as (#?) , as you want to match the octothorpe if present, not to match it always, but maybe zero times.

You correctly used a lexical filehandle for the input. Use a lexical filehandle for the output, too.

The preferred way of processing files, though, is to read them line by line if possible. You need to print to a temporary file, as you can't read a file and print to it at the same time.

#!/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 $!;
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]