Give a meaning to '&' in n-arg open case:

Give a meaning to '&' in n-arg open case:
  open($fh,"<&",$scalar);
  $scalar can be:
  - an integer which does "fdopen"
    open($fh,"<&",2); # like open($fh,"<&2")
  - something that will yield a file handle via sv_2io()
    useful for dup'ing anonymous handles.
    e.g.:
    open(my $fh,"<&",\*STDIN);
    open(my $dup,"<&",$fh);

p4raw-id: //depot/perlio@9298