{
    package Baz;

    sub new
    {
        my $class = shift;

        if ($class ne 'Baz') {
            if ($class->can('foo') != Baz->can('foo')) {
                die "Don't do that $class";
            }
        }

        bless {}, $class;
    }

    sub foo
    {
        'foo';
    }
    sub boo
    {
        'foo';
    }
}
{
    package Foo;
    our @ISA = 'Baz';

    sub foo
    {
        'Foo';
    }
    sub boo
    {
        'Foo';
    }
}
{
    package Boo;
    our @ISA = 'Foo';

    sub boo
    {
        'Boo';
    }
}
{
    package Goo;
    our @ISA = 'Baz';

    sub boo
    {
        'Boo';
    }
}

my ($a, $b, $c);

eval {
    $a = new Baz;
};
warn $@ if $@;
eval {
    $c = new Foo;
};
warn $@ if $@;
eval {
    $b = new Boo;
};
warn $@ if $@;
eval {
    $b = new Goo;
};
warn $@ if $@;