Not true I'm afraid. At a minimum most machines will have their primary interface and the hostname of its other interfaces within /etc/hosts.
Whilst you shouldn't underestimate the benefit of considering things that shouldn't happen in your environment, ie. access to root, that is not the reason for wanting to ensure we solely query DNS in this instance. DNS is my single source of truth and I do not wish to work with assumptions in order for my code to work.
Additionally it'll run across 100K+ hosts therefore you have to work with the lowest common denominator to ensure your code is portable across that number of hosts.
and force DNS lookup.
This isn't really a Perl discussion now though.