output

PLAY [localhost] ************************************************************** 

GATHERING FACTS *************************************************************** 
ok: [localhost]

TASK: [debug var=hash] ******************************************************** 
ok: [localhost] => {
    "var": {
        "hash": {
            "bar": "bar", 
            "foo": "foo"
        }
    }
}

TASK: [debug var=hash.foo] **************************************************** 
ok: [localhost] => {
    "var": {
        "hash.foo": "foo"
    }
}

TASK: [set_fact hash="{{ hash2 }}"] ******************************************* 
ok: [localhost]

TASK: [debug var=hash] ******************************************************** 
ok: [localhost] => {
    "var": {
        "hash": {
            "foo": "baz", 
            "quux": "quux"
        }
    }
}

TASK: [debug var=hash.foo] **************************************************** 
ok: [localhost] => {
    "var": {
        "hash.foo": "{{hash.foo}}"
    }
}

PLAY RECAP ******************************************************************** 
localhost                  : ok=6    changed=0    unreachable=0    failed=0   

playbook.yml

---

- hosts: localhost
  connection: local
  vars_files:
    - vars.yml
  vars:
    hash: "{{ hash1 }}"
  tasks:
    - debug: var=hash
    - debug: var=hash.foo
    - set_fact: hash="{{ hash2 }}"
    - debug: var=hash
    - debug: var=hash.foo

vars.yml

---

hash1:
  foo: foo
  bar: bar

hash2:
  foo: baz
  quux: quux