Ah, well that sucks. I would file a bug report. Since that doesn't work, I shall return to my first idea, which is to use Date::Calc. I didn't even know about Date::Format until you pointed it out.

#! /usr/local/bin/perl -w

use strict;
use Date::Calc 'Week_of_Year';

my ($y, $m, $d) = split m{/}, shift || '2007/12/31';
my ($week,$year) = Week_of_Year($y, $m, $d);

printf "%04d-W%02d\n", $year, $week;
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You might want to wrap that up in a little routine and be done with it. It will certainly be far faster than using Date::Manip