event_app.pl

use Mojolicious::Lite;
use Time::HiRes 'time';

# How long did receiving the request take?
hook after_build_tx => sub {
  my $tx = shift;
  my $before = time;
  $tx->req->on(finish => sub {
    my $req = shift;
    $req->headers->header('X-Time' => time - $before);
  });
};

get '/' => sub {
  my $self = shift;
  my $time = $self->req->headers->header('X-Time');
  $self->render(text => "Receiving the request took exactly $time seconds.");
};

app->start;