in reply to Explanation requested regarding negation of characters classes in perlretut
The point is that \D is the set of all characters that do not belong to \d, so 'a', 'C', '&' all belong to \D. Now similar for \W, the set of all characters not in \w, hence '&', ' ', etc.
So far so good, but [\D\W] is the union of the sets \D and \W, hence it will contain '&' and ' ', but also 'a' and 'C' since the latter are members of \D. That's definitely not equal to the set [^\d\w] that contains none of the characters in \d or \w. The set [^\d\w] definitely doesn't contain 'a' or 'C' that are members of [\D\W].
Hope this helps, -gjb-