my $count;

        *$sub = sub {
            @{ $self->{called_with} } = @_;
            $self->{called_count} = ++$called;
            if ($self->{side_effect}) {
                if (wantarray){
                    my @effect = $self->{side_effect}->(@_);
                    return @effect if @effect;
                }
                else {
                    my $effect = $self->{side_effect}->(@_);
                    return $effect if defined $effect;
                }
            }

            return undef if ! $self->{return};

            return ! wantarray && @{ $self->{return} } == 1
                ? $self->{return}[0]
                : @{ $self->{return} };
        };