Your variable is undefined because there wasn't an answer to the specific question you asked

From the Net::DNS::Resolver documentation:

Returns a "Net::DNS::Packet" object, or "undef" if no answers were found. If you need to examine the response packet whether it contains any answers or not, use the send() method instead.

This amended code uses send() instead:

use strict;
use warnings;
use Net::DNS;

my $domain = $ARGV[0];

#192.48.79.30 is j.gtld-servers.net

my $res = Net::DNS::Resolver->new(
        nameservers => [qw(192.48.79.30)],
        recurse     => 1,
        debug       => 1,
  );

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

my @authority = $packet->authority;

foreach my $rr (@authority) {
        print $rr->string, "\n";
}
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