$ua->on(start => sub ($ua, $tx) {...}) — real Mojo::UserAgent fires this before every request; every real call site in the legacy plugin corpus only uses it to set one or more static headers via $tx->req->headers->header(...), never anything request-specific (e.g. inspecting $tx->req->url), so it's faithfully enough modeled by running handler once, immediately, against a synthetic tx whose header setter writes into this instance's default headers (applied to every get/post call from then on) rather than actually re-invoking it per request. Only the "start" event is recognized — legacy's other events (error/finish/etc.) have no real use in this corpus and aren't modeled.