There basically is a premade Mojolicious solution with Mojolicious::Plugin::AutoReload, which does the automatic reloading already:

use Mojolicious::Lite;
plugin AutoReload => {};
get '/' => 'index';
app->start;
__DATA__
@@ index.html.ep
Hello world!
[download]

To use external files, you have to add an after_static hook, which is mainly what my App::Mojo::AssetReloader does, but LanX already knows about these :)