Whenever I need some operation involving dates, I immediately turn to Date::Manip. In this case, I think the code would go something like this:

$secs=time;
$date = &ParseDateString("epoch $secs");
$week=UnixDate($date,"%W");
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Note that you have:

     %U     week of year, Sunday as first day of week     - 01 to 53
     %W     week of year, Monday as first day of week     - 01 to 53
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