...Failed to serialize the request: hash- or arrayref expected (not a simple scalar, use allow_nonref to allow this) ...As I understand '/' failed to return the JSON which is true because I don't want to return JSON from '/' it should use 'template index'. Can anybody suggest how to accomplish this ?

If you want two serializers, you're going to need two apps, and a Plack::Builder to manage them , thats the lesson from Add serializer breaks Dancer2 app · Issue #833 · PerlDancer/Dancer2 · GitHub/Serial Serializer - making API writing easier | PerlDancer Advent Calendar

app.pl:
use MyApp;
use MyApp::JSONED;
use Plack::Builder;
builder {
    mount '/'    => MyApp->to_app;
    mount '/json' => MyApp::JSONED->to_app;
};
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