This can be done a lot easier using other Net::DNS::Packet and Net::DNS::RR methods ...

my $packet = $res->send($domain, 'MX');

foreach my $answer ($packet->answer) {

    next unless $answer->type eq 'MX';
    push @results, $answer->exchange;
}
[download]

Additionally, depending upon your requirements, it may be worthwhile to return a list of mail exchanges ($answer->exchange) and their respective preferences ($answer->preference).