It is my (limited) understanding that we do not represent the implementation of the data structure (e.g., hash), but rather the data itself. Web services should not make any assumptions about implementation at either end of the wire.
See page 99 of "Web Services with Perl" (Ray & Kulchenko, O'Reilly, 2003) for an example. The recommendation there is to define a compound type, with a parent element representing the hash, child elements named after the keys, and their text nodes containing the values.