I think the initial test in selfcheck should be:

sub selfcheck {
    my $self = shift;
    die "..." if ref $self ne __PACKAGE__;
    # lineage check...
}
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As you have it, the test would always pass because __PACKAGE__ is the package that the sub was compiled in (Foo::Bar) regardless of anything else (see perlmod).

If I were a "malicious subclasser" faced with this, I could redefine selfcheck (I think), and then it's smooth sailing. What you could do is put that in a lexical subref, then it would stay private.

package Foo::Bar;
# ...
my $selfcheck = sub {
    my $self = shift;
    die ...
};

sub method {
    my $self = shift;
    $selfcheck->( $self );

    # ...
}
[download]

I'm not sure where to go from there. Putting that check seems like a lot of work for the goal, but I guess it could be worse.