It turns out that if one uses pkg_add in FreeBSD, the
64-bit version of mhonarc is stale. You get mhonarc6.18 all
right, but Earl Hood has re-written the scripts since then and
made them work perfectly. Many thanks!
I figured out what needed to be edited in the old
scripts but was still having some difficulty so I completely
blew away the installation of mhonarc and this time re-installed
it from the most recent FreeBSD ports collection. That fixed
everything as the mhamain.pl script as well as mhopt.pl and
readmail.pl also contain the "defined" directive that modern
perls don't like.
Anyway, the problem is solved. The 64-bit system that
had worked all along had been built with mhonarc from a port. I
had just gotten lazy and used the package and payed the lazy
price. That's pretty bad as make install is actually less typing
than pkg_add -r mhonarc.