sub plugin {
  my $self=shift;
  unless $self->{plugin} {
    my $ok = eval "require $Plugin_Package;";
    die("error load $Plugin_Package\n$@") unless $ok;
    $self->{plugin} = $Plugin_Package->new();
  }
  return $self->{plugin};
}

#...

sub get_key {
    # ...
    $self->plugin()->some_method();
    # ...
}