Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open. This is mainly useful for constructing
bitmaps for select and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
(Filehandles connected to memory objects via new features of open may
return undefined even though they are open.)