Looks like Perl 4 is being invoked on mhonarc. You need Perl 5.
This turned out to be the problem, but I never could have figured it out on
my own. I finally broke down and told my ISP that I couldn't figure out why
mhonarc run from procmail would produce Perl 4 type syntax messages, but
not from the command line. Turns out that procmail is on a different
machine than I ordinarily telnet to and is set to run Perl 4. Don't ask me
why. In any case, I got past that problem and am now trying to run my own
downloaded version of 2.4.3 from my own directory using procmail rather
than their older beta version. The very first time I ran it, I got the
following error message:
Can't modify keys in scalar assignment at
/users/lnp3/MHonArc2.4.3/lib/mhutil.pl line 122, near "$hs;"