...
my %count;

$count{$_}++ for @dates;
print "$count{$_} times $_\n" for sort {$count{$b} <=> $count{$a}} key
+s %count;
[download]

Prints:

Content-Type: text/html; charset=ISO-8859-1

4 times 02/20/2009
2 times 02/01/2009
2 times 01/31/2009
1 times 01/01/2009
[download]