Very nice... A couple extra ideas, which may or may not already be in the changes you made before sending it to CPAN:

- Change $offset from my to our (or add a method for setting it) so that tests can, e.g., do something now (at an offset of +0) which expires in an hour, then set offset to +3600 and check whether it has expired.

- Add an our $fixed_time which, if defined, returns that time (instead of actual time + offset) for all calls. This would be useful for cases where you're testing that a sub stores the time correctly and don't want to have to worry about whether the system clock rolled over from 12345678 to 12345679 between when the time is stored and when you test the stored value's correctness.