open my $olderr, '>&STDERR'; # save STDERR
my ($fh, $fn) = tempfile();
open STDERR, '>&', $fh;

printf "fileno STDERR: %d\n", fileno(STDERR);
print STDERR "foo!";
test_err();
system "perl -e die";
open STDERR, '>&', $olderr; # reset STDERR
close $fh;
open my $e, '<', $fn;
my $err = <$e>;
print "|$err|\n";
__END__
__C__
void test_err() {
    FILE *err = fdopen(2, "a+");
    (void)fprintf (err, "ERROR");
}

# fileno STDERR: 2
# |foo!Died at -e line 1.
# |
# ERROR