use File::Temp 'tempfile';

print "Trying oldway...\n";
my $oldway = oldway(); # should not display anything
print "Finished with oldway.\n";
print $oldway;

print "Testing STDERR...\n";
print STDERR "(STDERR) can you hear me?\n";

print "Trying newway...\n";
my $newway = newway(); # should not display anything
print "Finished with newway.\n";
print $newway;

print "Testing STDERR...\n";
print STDERR "(STDERR) can you hear me now?\n";

sub oldway {
  my ($fh, $filename) = tempfile(UNLINK => 1);
  open(OLDERR, ">&STDERR") || die "Can't save STDERR, $!"; # save STDERR
  open(STDERR, '>', $filename) || die "Can't redirect STDERR to the temp file, $!";
  select(STDERR); $| = 1;
  print STDERR "oldway says 'hi'\n";
  open(STDERR, ">&OLDERR"); # restore the original STDERR
  select(STDOUT);
  my $oldway = slurp_file($filename);
  return $oldway;
}

sub newway {
  my ($fh, $filename) = tempfile(UNLINK => 1);
  *oldfh = select STDERR; # save STDERR
  open(STDERR, '>', $filename) || die "Can't redirect STDERR to the temp file, $!";
  select(STDERR); $| = 1;
  print STDERR "newway says 'hi'\n";
  open(STDERR, ">&oldfh"); # restore STDERR
  select(STDOUT);
  my $newway = slurp_file($filename);
  return $newway;
}

sub slurp_file {
  my $filename = shift;
  my $slurp;
  open(TXT, '<', $filename) || die "Can't read the file: $filename, $!";
  {
    local $/ = undef;
    $slurp = <TXT>; # slurp!
  }
  close(TXT);
  return $slurp;
}

__DATA__
Trying oldway...
Finished with oldway.
oldway says 'hi'
Testing STDERR...
(STDERR) can you hear me?
Trying newway...
Finished with newway.
newway says 'hi'
Testing STDERR...
(STDERR) can you hear me now?