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. Martin McCormick