fileno function
Return file descriptor from filehandle
Returns the file descriptor for a filehandle. This is 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";
}