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;

##</code><code>##

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

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

rename( $hosts, "$hosts.old" );

open IN, "$hosts.old" or die "Can't read $hosts.old $!\n";
open OUT, ">$hosts" or die "Can't write $hosts $!\n";
while (my $old = <IN>) {
    print OUT $old unless $old =~ m/\Q$ip/;  
}
print OUT $line;  # add the new record
close IN;
close OUT;

##</code><code>##

    $old =~ s/\Q$old_ip\s+/$new_ip /;
    print OUT $old;

##</code><code>##

C:\>perl -pi.bak -e "s/this/that/g" <file>