printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT);

open $old_stdout, ">&STDOUT" or die "open: $!";
# _don't_ explicitly close STDOUT here

local(*RH, *WH);
pipe RH, WH;
open STDOUT, ">&WH" or die "open: $!";

printf STDERR "after:  fileno(STDOUT)=%d\n", fileno(STDOUT);