spamc/spamd doesn't work after upgrading to 5.1

[Kirk Strauser]

I use SpamAssassin 2.55 from the mail/p5-Mail-SpamAssassin port.  It's been
working for months under FreeBSD 4.8, but recently quit when I upgrade to
5.1.  spamd is running and I can connect to its port with netcat or telnet.
spamc still seems to connect to spamd, but exits immediately without marking
mails passing through it.  That is, if I try to parse one of SpamAssassin's
sample files into spamc, then spamc prints out the contents of the file,
unmodified, without any observable pause:

    root at kanga:~# time spamc < /usr/local/share/doc/p5-Mail-SpamAssassin/sample-nonspam.txt > foo
    real    0m0.056s
    user    0m0.001s
    sys     0m0.012s
    root at kanga:~# cmp /usr/local/share/doc/p5-Mail-SpamAssassin/sample-nonspam.txt foo

I'm at a loss.  /var/log/maillog shows the connection, but that's about the
only noticeable side effect:

    Jul 19 15:35:05 kanga spamd[83988]: connection from localhost [127.0.0.1] at port 56035

I have no idea what to test next.  Any troubleshooting suggestions?
-- 
Kirk Strauser
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