%db = (
  year => {
    month => {
      day => {
        foo => 1, bar => 1, baz => 1, and so on..., 
      }
    }
  }
);

if year, month, day
  I want 'n' keys from $db{year}{month}{day}
if year, month
  I want 'n' latest keys from the days from $db{year}{month}
if year
  I want 'n' latest keys from the days from $db{year}
else no date defined
  I want 'n' latest keys from %db