As a special case the 3 arg form with a read/write mode and the
third argument being "undef":

    open(TMP, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file.  Also using
"+<" works for symmetry, but you really should consider writing
something to the temporary file first.  You will need to seek()
to do the reading.

##</code><code>##

If FILEHANDLE is an undefined scalar variable (or array or hash
element) the variable is assigned a reference to a new anony-
mous filehandle,