my %month_sort = 
  ( 'JAN' =>  1, 'FEB' =>  2, 'MAR' =>  3,
    'APR' =>  4, 'MAY' =>  5, 'JUN' =>  6,
   ... etcetera ...
   );

sub sort_cmp {
   my ($a, $b) = @_;
   return
      ( substr($a, 3, 2) cmp substr($b, 3, 2) )
      ||
      ( $month_sort->{substr($a, 0, 3)} <=> 
        $month_sort->{substr($b, 0, 3)}
      );
}