Here is a test case that reliably demonstrates the problem on 5.8.0. If you move the second "use base" to before the preceeding "use fields" the test will succeed. Internally %Bar::FIELDS is getting populated with duplicate values which is causing the trouble.

package Foo;

use strict;
use fields qw(Foo_field);

sub new ($) {
    return fields::new(shift);
}

package Bar;

use strict;
use fields qw(Bar_field);
use base qw(Foo);

package main;

use Test::More 'no_plan';

my Bar $obj = new Bar();

ok ($obj, "got instance");

ok ($obj->isa("Bar"), "it's a Bar");
ok ($obj->isa("Foo"), "it's a Foo");

ok ($obj->{Bar_field} = 2, "Can assign Bar_field");
ok ($obj->{Foo_field} = 3, "Can assign Foo_field");

is ($obj->{Bar_field}, 2, "Bar is assigned");
is ($obj->{Foo_field}, 3, "Foo is assigned");
[download]