See Mail::Box::Locker
See SYNOPSIS in Mail::Box::Locker
This locker object is uses File::FcntlLock, and was contributed by Jim in Aus. It is close to Mail::Box::Locker::POSIX, but does work on more systems, for instance Darwin.
You will need to install File::FcntlLock separately: there is no dependency to it by the MailBox distribution.
See DESCRIPTION in Mail::Box::Locker