So that means you must write your own comparison subroutine, as described in sort. You'll also need to specify ordering on months, since MMM format doesn't naturally sort. Something like:

my %MMM_order = (
    Mar => 3,
    Apr => 4,
    May => 5,
    Dec => 12,
);
sub cmp_MMMYY {
    my ($a_mon, $a_yr) = $a =~ /(.{3})(\d{2})/;
    my ($b_mon, $b_yr) = $b =~ /(.{3})(\d{2})/;

    $a_yr  cmp $b_yr
       or
    $MMM_order{$a_mon}  <=> $MMM_order{$b_mon};
}
[download]

Given the content of your post convert date format from YYYY-MM-DD to YYYYMMDD, I should mention that this sort would be much more natural in ISO 8601, and there are conversion methods discussed in the response to that post.