in reply to d-mmm-yyyy to DOY (day of year)
Also, if you do find that you need to “install something” in order to address your problem ... by all means, stop and “look for a Plan-B.” There are many date-manipulation packages in Perl ... literally, hundreds of them. Something is already going to be installed, that you can use to get this job done. See for example perldoc perllocal. You don’t need to smash down this particular door, to get to where you need to be, given such a simple requirement.
Here’s a handy one-liner: perl -e 'use Date::Manip;' ... enter that on the shell command-line exactly as shown. If that command succeeds (no nasty messages spew out), then you know that this module Date::Manip in this case) is available on your system, so perldoc Date::Manip to learn how it works.