How would your approach of

$rtt_max ||= $default_timeout;
[download]

have worked out if $rtt_max was -1 ?

Also, before tieing the maturity of a language to problems with its programs, have you checked if and whether time is monotonic?

Personally, I would assume that the same problematic constellation would happen with Perl, and mocking both the time and the round trip times from DNS queries in a realistic manner only makes sense if your code base is small and you already suspect a problem in that location IMO.