fileno

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.