Date::Tie can do the math part:

use strict;
use Date::Tie;

tie my %date, "Date::Tie";
# 3rd Sunday in March
$date{month} = 3;    # march is 3
$date{day} = 1;      # start counting form march first
$date{weekday} = 7;  # monday is 1, sunday is 7
$date{week} += 2;    # advance 2 weeks
print "3rd Sunday in March is $date{month}-$date{day}\n";

# 2nd Friday in April
$date{month} = 4;    # april is 4
$date{day} = 1;      # start counting form april first
$date{weekday} = 5;  # monday is 1, friday is 5
$date{week} += 1;    # advance 1 week
print "2nd Friday in April is $date{month}-$date{day}\n";
[download]

Output:

3rd Sunday in March is 03-17
2nd Friday in April is 04-12
[download]

update: Date::Tie is fast