use Date::Calc qw(Day_of_Week Week_Number Day_of_Year);
# you have $year, $month, and $day
# $day is day of month by definition
$wday = Day_of_Week($year, $month, $day);
$wnum = Week_Number($year, $month, $day);
$dnum = Day_of_Year($year, $month, $day);