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:

if (fileno(THIS) == fileno(THAT)) {
    print "THIS and THAT are dups\n";

(Filehandles connected to memory objects via new features of open may return undefined even though they are open.)