I think that the code below will do what you want. You do need to make sure that new is called. 
{
    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 $@;

[download]
If you just want to make sure that Baz::foo is being called from code you control you can just do: 
$class->Baz::foo();

[download]
I can't think of anything that you could do in a BEGIN block that would help with this problem.
-- gam3
A picture is worth a thousand words, but takes 200K.

In reply to Re: checking a method is not subclassed by gam3
in thread checking a method is not subclassed by water

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