XMLHttpRequest cannot load http://127.0.0.1:5000/api/v1/greeting.
No 'Access-Control-Allow-Origin' header is present on the requested resource.
Origin 'null' is therefore not allowed access.

lib/D2/Ajax.pm

header 'Access-Control-Allow-Origin' => '*';

Let's create v2 of the API

snippets/2/lib/D2/Ajax.pm
get '/api/v2/greeting' => sub {
    header 'Access-Control-Allow-Origin' => '*';
    header 'Content-Type' => 'application/json';
    return to_json { text => 'Hello World' };
};

Try: v2