The best way to drill that down is to follow the classes of the objects.

1. A Net::DNS::Packet object has the object method answer (You've already figured that out).
2. According to the docs, that answer is a list of Net::DNS::RR objects. These have a type method, which will provide you with 'PTR' (I'm assuming that from your dump).
3. Finally, Net::DNS::RR::PTR objects have a method ptrdname, which provides you with a the string '20.bl.bot.semrush.com'. You get the same result from calling $answer->rdstring, which works for all types of RR records.

In code:

use 5.020;
my @answers = $obj->answer;
for my $answer (@answers) {
    if ($answer->type eq 'PTR') {
        $ptrdname = $answer->ptrdname;
    }
}
say $ptrdname;
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