This lets you implement filesystems in perl, through the FUSE
(Filesystem in USErspace) kernel/lib interface.

FUSE expects you to implement callbacks for the various functions.

In the following definitions, "errno" can be 0 (for a success),
-EINVAL, -ENOENT, -EONFIRE, any integer less than 1 really.

You can import standard error constants by saying something like "use
POSIX qw(EDOTDOT ENOANO);".

Every constant you need (file types, open() flags, error values, etc)
can be imported either from POSIX or from Fcntl, often both. See their
respective documentations, for more information.