bad free after iter->to_arrayref

[Kevin Ryde <user42 zip com au>]

The program below gets an error for me,

    Attempt to free unreferenced scalar: SV 0x83c10a8, Perl interpreter: 0x8150008.

It seems to have something to do with returning a reference as data in
an iter.  Are you allowed to do that?  I saw "Will not persist" in the
Gtk2::TreeModel docs, but couldn't tell what it meant.

package MyModel;
use strict;
use warnings;
use Gtk2;

use Glib::Object::Subclass
  Glib::Object::,
  interfaces => [ Gtk2::TreeModel:: ];

sub ITER_NTH_CHILD {
  my ($self, $iter, $n) = @_;
  my $ref = [ 123 ];
  return [ 999, 0, $ref, undef ];
}

package main;
use strict;
use warnings;
use Gtk2;
my $model = MyModel->new;

my $iter = $model->iter_nth_child (undef, 5);
$iter->to_arrayref(999);