DateTime::Lite versions before 0.8.1 for Perl allow CPU exhaustion via a per-day loop when adding seconds.
_normalize_leap_seconds crosses day boundaries one day at a time, so add(seconds => $n) on an object with a non-floating time zone takes about |$n|/86400 iterations, and nothing limits $n. The XS implementation and the pure-Perl fallback both have the loop.
An attacker who controls the number of seconds added can keep the process busy for as long as they choose.