See Mail::Box::Locker
See SYNOPSIS in Mail::Box::Locker
This locker object is created by the folder to get an exclusive lock on the file which contains the data using the kernel's POSIX facilities. This lock is created on a separate file-handle to the folder file, so not the handle which is reading.
WARNING: Not all platforms support POSIX locking (via fcntl) and not always in the same way. This implementation does not use XS to access the structure of fcntl(): it is better to use the ::FcntlLock which does. No, this implementation "guesses" the location of the bytes.
See DESCRIPTION in Mail::Box::Locker