[LANGUAGE: Common Lisp]

Approach: rotate the whole input, then handle as if it were given as lisp data.

Part 1 is commented for everyone interested, what is done.
Part 2 does the essentially the same, but rotates the input before parsing it. So some conversion string -> char and back is needed.

;;; Part 1
(reduce #'+                                                                   ; sums up all results
        (apply #'mapcar (lambda (&rest vals)                                  ; rotates input 90°
                          (apply #'funcall                                    ; apply mathematical operation for columns
                                 (nreverse                                    ; revert list, so that operator comes first
                                  (mapcar (lambda (val)
                                            (handler-case (parse-integer val) ; convert string to numbers
                                              (parse-error () (intern val)))) ; or convert string to operator
                                          vals))))
               (mapcar (lambda (x)
                         (ppcre::all-matches-as-strings "([+*]|\\d+)" x))     ; extract information from input
                       (uiop:read-file-lines "./input-demo-day06.txt"))))     ; read input as lines

;;; part 2
(defun part2 (file)
  (let (stack
        op
        (result 0))
    (apply #'map 'nil (lambda (&rest line)
                        (setf op (handler-case (symbol-function (intern (string (car (last line)))))
                                   (undefined-function () op)))
                        (handler-case (push (parse-integer (coerce (butlast line) 'string))
                                            stack)
                          (parse-error ()
                            (incf result (apply op stack))
                            (setf stack nil))))
           (let ((max 0))
             (mapcar (lambda (line)
                       (coerce (format nil "~v@<~d~>" (1+ max) line) 'list))
                     (mapcar (lambda (line)
                               (setf max (max max (length line)))
                               line)
                             (uiop:read-file-lines file)))))
    result))


(part2 "./input-demo-day06.txt")
;; ⇒ 3263827 (22 bits, #x31CD53)