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