#!/usr/bin/perl
use strict;
use warnings;
use Date::Manip;
use feature 'say';

my $datestr = ParseDate("01/01/2016");
say UnixDate($datestr,"%J");

Date_Init("Jan1Week1=1");
say UnixDate($datestr,"%J");

__END__

$ perl test.pl
2015-W53-5
2016-W01-5