Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
Neither Date::Manip nor Date::Calc deal with Julian days. Instead, there is an example of Julian date calculation that should help you in http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz .