My instincts tell me that this is probably a subtle error with your code, instead of Mail::Audit.   So, I would suggest that you start with this statement and trace it back to the method-calls in your software which lead up to that call.   Trace the variable (whose _log method is ultimately being called) back to its point of origin within your code.

Since the problem occurs only rarely, it is most certainly “data-driven.”   That is to say, some subtle and rarely-occurring characteristic of the message itself.   I can’t predict what the software should be doing with these particular messages, but maybe it should not be logging them, or even processing them at all.   Every single one of the messages which are triggering this problem ought to have some characteristic that every single other one does not.   This is where your debugging efforts should begin.   As a rule of thumb, presume that the CPAN module does not contain an error, until you can demonstrate otherwise.   This will almost certainly prove to be the case.