Long story short... I had opened the Mail::Audit logfile as "-", because I wanted it to use stdout, which in my procmailrc I had directed to the proper log.
But RJBS recently changed Mail::Audit from using the two-arg open for this name to the three-arg open for this name, and this was only apparent once I had used the CPAN diff tools (only in the source, and not documented, sadly).
Yes, I had created a 30MB logfile named "-" in my home directory. After carefully removing that file, and using an explicit filename for logfile, all was good.
But this is a heads-up for anyone else who might have presumed that "-" means stdout in Mail::Audit... you might be logging somewhere odd right now. :)
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.