As apl suggests, there are several modules on CPAN to manipulate dates and times. One is DateTime which you could use like this:

use DateTime;

my $d = new DateTime(year => 2008, month => 3, day => 31);
my $yesterday = DateTime->today()->add(days => -1);
while (1) {
  my $year = $d->year;
  my $month = $d->month;
  my $day => $d->day;
  ... download file for $year, $month, $day ...
  last if ($d->ymd('/') eq $yesterday->ymd('/'));
  $d->add(days => 1);
}
[download]