$ perldoc -f fileno
   fileno FILEHANDLE
       Returns the file descriptor for a filehandle, or undefined if
       the filehandle is not open.  This is mainly useful for con-
       structing 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.)
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Dave.