You might have some that read

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

There are, as always, many ways to do it - one:

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

That will work only if the opening curly after a zone declaration is on the same line.

BTW, you need a semicolon after every closing curly in your zone file ( }; ).

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}