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};
}