GrandFather, the problem seems to be this:

%partprice = (
  1 => {
    '2005-01-01' => 1.99, # new year special.
    '2005-01-02' => 8.99, # back to normal.
    '2005-02-15' => 9.99, # price bump.
    '2005-04-01' => 9.49, # on sale.
    '2005-04-08' => 9.99, # sale's over.
  },
);
[download]

The question from this is: what is the cost of item #1 for a purchase made on March 18th, 2005? For that, you have to scan through looking for the last date key that is less than or equal to the desired date.

If, however, you had an ordered hash, you could do a binary search for it. Or if you had a tree or something where the date keys were stored in an order, you could look it up faster.