fileno filehandle
Returns the file descriptor for a filehandle.
(A file descriptor is a small integer, unlike the filehandle, which
is a symbol.) It returns undef if the handle is not open.
It's useful for constructing bitmaps for select, and for
passing to certain obscure system calls if syscall is
implemented.
It's also useful for double-checking that the open function
gave you the file descriptor you wanted.