sub call {
    my $self = shift;
    my $env  = shift;

    my $req = Plack::Request->new($env); # nicer than fiddling direclty with env

    if ($req->method eq 'POST' && $req->parameters->get('op') eq 'FOO') {
        # TODO check CSRF
        $env->{PATH_INFO} = '/some/error/page';
    }

    return $self->app->($env);
}