sub sortKeys {
    my ( $l_mo, $l_yr ) = $a =~ /(\D+)(\d+)/;
    my ( $r_mo, $r_yr ) = $b =~ /(\D+)(\d+)/;
    my $left  = $l_yr . $months->{$l_mo};
    my $right = $r_yr . $months->{$r_mo};
    return ( $left cmp $right );
}