Thank you! That won't work in my real situation, because I won't be calling a class method like that, but you made me realise that instead of __PACKAGE__, I just need to call ref $self, which will inherit properly. D'oh:

    my $self = shift;
    my $factory_class=ref $self;
    *Foo::bar = sub { return $factory_class->instance; };
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I definitely am creating a closure now, but I don't think it will matter: it's only a simple scalar that gets unnaturally prolonged.

Thanks.