zone "10.in-addr.arpa" {
    type forward;
    forwarders {
        10.1.2.3;
        10.1.2.4;
        10.1.2.5;
    };
};

##</code><code>##

my $comment = 0;
my $block   = 0;
while (<HAN>) {
    /^zone\s+"$targetdomain"/ and $comment++;
    if($comment) {
        $block += () = /(\{)/g;        # add the number of { matches to $block
        s!^!// ! if $comment && $block;
        $block -= () = /(\})/g;        # substract the number of } matches
        $comment = 0 unless $block;
    }   
    print;
}