I agree with Randal. I think Larry said something about livingrooms and shotguns on this topic. However, you might try something in your method like this:

sub foo {
  my $self = shift;
  die "Get out of my class" unless ref($self);  # not called as method
  die "Get out of my class" unless $self->isa('My::Class');
  
}
[download]

Above code for illustrative purposes only. Certaily could be done a lot better/cleaner.