The discussion of date parsing in Time::Piece:

  my $t = Time::Piece->strptime("Sun 3rd Nov, 1943",
                                "%A %drd %b, %Y"); 
  print $t->strftime("%a, %d %b %Y");

Outputs:

  Wed, 03 Nov 1943

(see, it's even smart enough to fix my obvious date bug)
[download]

suggests that this behaviour is an intended feature. It also suggests a way that you can verify dates:

use strict;
use warnings;
use Time::Piece;
my $format = "%d-%m-%Y";
while (my $date = <DATA>) {
    chomp $date;
    printf("%s %s legitimate\n", $date,
           (verify_date($date, $format) ? "is" : "is not"),
          );
}
sub verify_date {
    my ($date_in, $format) = @_;
    my $t = Time::Piece->strptime($date_in, $format);
    my $date_out = $t->strftime($format);
    return $date_in eq $date_out ? 1 : 0;
}

__DATA__
28-02-2004
29-02-2004
30-02-2004
28-02-2006
29-02-2006
30-02-2006
[download]

which leads to:

28-02-2004 is legitimate
29-02-2004 is legitimate
30-02-2004 is not legitimate
28-02-2006 is legitimate
29-02-2006 is not legitimate
30-02-2006 is not legitimate
[download]