sub discard_sortkey { unpack 'x[nn]A*' };
sub prepend_yearmonth {
    my( $alpha, $num ) = m[^(\S+?)\s*(\d+)$];
    $num += 2000 if $num <= 49;
    $num += 1900 if $num <= 99;
    pack 'nnA*', $num, $months{ $alpha }, $_;
}
print for map{ discard_sortkey } sort map { prepend_yearmonth } <DATA>;