It depends what you mean. Given the calendar:

 S  M  T  W  T  F  S
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30

which "week" is the 12th in? Is it the 2nd or 3rd week? If you count a week as 7 consecutive days, starting with the week of the 1st through 7th, then the 12th is in the second week. If, on the other hand, you can a week as the days Sunday through Saturday, then the 12th is in the third week.

Please clarify your question, anonymous one, so that we can better direct your efforts.

Either solution can be devised simply through the use of the standard Time::Local module. (I eschew those everything-but-the-kitchen-sink Date/Time modules.)