sub selfcheck {
    my $self = shift;
    die "..." if ref $self ne __PACKAGE__;
    # lineage check...
}

##</code><code>##

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

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

    # ...
}