What Corion said and this is a nice snippet to help with the issue-

  builder {
      enable "Rewrite", rules => sub {
          return 204 if $_ eq "/favicon.ico";
      };
      enable "Session";
      $app;
  };
[download]

You might need to cpanm Plack::Middleware::Rewrite first; Plack::Middleware::Rewrite.