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. :)
-- Randal L. Schwartz, Perl hacker
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.