open STDIN, '<&', $fh
        or do { warn "set STDIN = \$fh=\\$fh: $!"; return undef; };

##</code><code>##

    open my $oldin, "<&STDIN"
        or do { warn "dup STDIN to \$fh=\\$fh: $!"; return undef; };

##</code><code>##

C:>perlver t\03_input_processing.t

       ------------------------------------------------------
     | file                    | explicit | syntax | external |
     | ------------------------------------------------------ |
     | t\03_input_processing.t | v5.6.0   | v5.6.0 | n/a      |
     | ------------------------------------------------------ |
     | Minimum explicit version : v5.6.0                        |
     | Minimum syntax version   : v5.6.0                        |
     | Minimum version of perl  : v5.6.0                        |
       ------------------------------------------------------