Turns out there are already sun time calculation modules in the Astro name space. However, there is a little more back story than was presented initially that maybe excuses the lack of a CPAN search up front.

I have a son in the Navy and recently he was tasked with preparing a passage plan which, among other things, entails figuring out when sun rise and sun set are for each day of the journey. That turns out to be a somewhat non-trivial problem because the boat moves. They use an Excel spread sheet to do a lot of the work. But for sun times they enter times from an almanac, calculate a position at the entered time, then iterate.

That seemed entirely too much work (and error prone) so I hunted up an algorithm on the web, implemented it using Perl to check I had it right, then reimplemented it in the spread sheet. When I needed code for the transit calculation I already had it lying around so I didn't bother with a CPAN search.