When I use SpamAssassin with Mail::Audit, I get a you cannot use body() error.
Using a version of Mail::Audit later than 2.1 at least, this error will appear when you call the SpamAssassin rewrite_mail method:
This is caused by an API which changed in Mail::Audit. Either wait for the next release of SpamAssassin for a workaround, or work around it yourself by using the nomime option when constructing the Mail::Audit object:
(Thanks to Jake Edge for the answer).