Nice example to understand ioloop.

ioloop.pl

my $t = Test::Mojo->new;

sub f2 {
  my $cb = shift;
  my $d = Mojo::IOLoop->delay(
    sub {
      warn 'f2_1', $/;
      shift->pass;
    },
    sub {
      warn 'f2_2', $/;
      $cb->();
    }
  );
  $d->on(finish => sub {
      warn 'FINISH f2', $/;
    });
  $d->wait;
  my $subscribers = $d->subscribers('finish');
  warn 'Len2 : ', scalar @$subscribers, $/;
}

sub f1 {
  my $cb = shift;
  my $d = Mojo::IOLoop->delay(
    sub {
      warn 'f1_1', $/;
      f2(shift->begin);
    },
    sub {
      warn 'f1_2', $/;
      $cb->();
    }
  );
  $d->on(finish => sub {
      warn 'FINISH f1', $/;
    });
  $d->wait;
}

my $d = Mojo::IOLoop::Delay->new;
$d->steps(
# my $d = Mojo::IOLoop->delay(
  sub {
    warn 11111;
    f1(shift->begin);
  },
  sub {
    my ($delay) = @_;
    warn 2222;
    $delay->pass;
  },
  sub {
    warn 3333;
    return;
  }
);
my $subscribers = $d->subscribers('finish');
warn 'Len: ', scalar @$subscribers, $/;
$d->on(finish => sub {
    warn '!!FINISH!!', $/;
  });
$d->wait;

#Mojo::IOLoop->start;

done_testing;