app.pl

use Mojolicious::Lite -signatures;

hook after_build_tx => sub ($tx, @) {
  $tx->req->max_message_size(110);
  $tx->req->content->on(body => sub ($) {
    my $url = $tx->req->url;
    return unless $url->path eq '/long_ok';
    app->log->debug("max message size extended for $url");
    $tx->req->max_message_size(1e8);
  });
};

under '/' => sub ($c) {
  return 1 unless $c->req->is_limit_exceeded;

  $c->render(status => 400, text => 'Request was too large');
  return 0;
};

post '/*catchall' => {catchall => ""} => sub ($c) { $c->render(json => {ok => \1}) };

app->start;

run as perl app.pl get -v -M POST -c 'XXXXXXXXXXXX' / vs /long_ok