As of v5.32 these monitors definitely don't work. As a previous poster noted,
they're calling "&hostname", which will pass in the call arguments from the
call site (propagating @_) which means they might fail. And these calls are
sometimes inside of eval{} meaning the error is lost and it seems like things
worked.
Seems it may affect: imap.monitor, nntp.monitor, pop3.monitor, smtp.monitor
I'll look at sending a patch upstream, but I think mon is pretty stable these
days. It'd be nice to see this fixed in Debian if possible.
--
rjbs