Well, I'm not only new to Perlmonks, but even newer to Lisp, as I just started to learn it. So, since a lot of different solutions have been posted, I thought I post my Common Lisp version (there's probably a better way to do it and I hope there isn't a bug):

  (defun roman_to_dec (roman)
    (if (not (equal roman ""))
      (let ((rtoa '((#\M . 1000)
                    (#\D . 500)
                    (#\C . 100)
                    (#\L . 50)
                    (#\X . 10)
                    (#\V . 5)
                    (#\I . 1))))
        (reduce #'(lambda (a b) (- (+ a b) (* (mod a b) 2)))
                (map 'list
                     #'(lambda (c) (cdr (assoc c rtoa)))
                     (string-upcase roman))))))
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Update: Ok, the

(if (not (equal roman ""))

could probably be removed, because no error checking is necessary regarding to the description, and it is halfhearted to boot.