use strict;
use warnings;

# Update $fname re-runnably.
# $fcontents is either a string or a reference to an array of lines.
sub atomic_update
{
   my ( $fname, $fcontents ) = @_;
   print "updating '$fname'...";
   my $bak = $fname . $$ . '.tmp';
   -e $bak and ( unlink($bak) or die "error: unlink '$bak': $!" );
   open( my $fh, '>', $bak ) or die "error: open '$bak': $!";
   print {$fh} ref($fcontents) ? @{$fcontents} : $fcontents
      or die "error: print '$bak': $!";
   close($fh) or die "error: close '$bak': $!";
   rename( $bak, $fname ) or die "error: rename '$bak' '$fname': $!";
   print "done.\n";
}

my $hostfile = shift || "C:/Windows/System32/drivers/etc/hosts";
print "hostfile : $hostfile\n";

open(my $In, '<', $hostfile) or die "error: open '$hostfile': $!";
my @Output = <$In>;
close $In;

# If 10.10.1.2 is commented out, uncomment it.
# If it is not commented out, comment it out.
my $nchanges = 0;
for (@Output)
{
   if ( s/^#(?=10\.10\.1\.2\b)// )
   {
      ++$nchanges;
      next;
   }
   if ( s/^(?=10\.10\.1\.2\b)/#/ )
   {
      ++$nchanges;
      next;
   }
}
if ($nchanges > 0)
{
   atomic_update( $hostfile, \@Output );
}
print "There were $nchanges changes made to file '$hostfile'\n";