This is an excerpt from the latest version perlfaq4.pod, which
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4.16: How can I find the Julian Day?

(contributed by brian d foy and Dave Cross)

You can use the "Time::JulianDay" module available on CPAN. Ensure that
you really want to find a Julian day, though, as many people have
different ideas about Julian days. See
http://www.hermetic.ch/cal_stud/jdn.htm for instance.

You can also try the "DateTime" module, which can convert a date/time to
a Julian Day.

$ perl -MDateTime -le'print DateTime->today->jd'
2453401.5

Or the modified Julian Day

$ perl -MDateTime -le'print DateTime->today->mjd'
53401