- fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. This is mainly useful for constructing bitmaps for
selectand 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
openmay return undefined even though they are open.)