You invoke an anonymous sub with ->(). Your sub would be invoked with $new_resolvers{"dns-extern"}[0]->($ip_address). The way you have your loop written, you'll have to tell the difference between a scalar and a sub though. You could rewrite the loop as:

   print "getting new resolvers for dns server $dns_server\n";
   for my $resolver ( @{$new_resolvers{$dns_server}} ) {
     if (ref $resolver) {
       print "$_\n" for $resolver->($ip);
     } else {
       print "$resolver\n\n";
     }
   }
   print "\n";
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I don't know where $ip is supposed to come from, but that's the way you have your anonymous sub written.