my $hosts = "C:/windows/system32/drivers/etc/hosts";

print "Line to add to hosts (x.x.x.x domain.com): ";
my $line = <STDIN>;

rename( $hosts, "$hosts.old" );  # effectively make backup

open FILE, ">$hosts" or die "Can't write $hosts $!\n";
print FILE "127.0.0.1 localhost\n$line";
close FILE;