ansible-map.yaml

#!/usr/bin/env ansible-playbook
- hosts: localhost
  vars:
    my_env:
      XXX_FOO: foo_value
  tasks:

    - set_fact:
        my_env: "{{ my_env | combine({ 'XXX_BAR': 'bar_value'}) }}"

    - environment: "{{ my_env }}"
      block:
        - shell: |
            set | grep XXX
          register: result

        - debug:
            var: result

And output:

TASK [debug] ******************************************************************************************************************************************************************************************************************************************************
Thursday 31 January 2019  11:19:50 +0000 (0:00:00.270)       0:00:00.995 ******
Thursday 31 January 2019  11:19:50 +0000 (0:00:00.270)       0:00:00.994 ******
ok: [localhost] => {
    "result": {
        "changed": true,
        "cmd": "set | grep XXX",
        "delta": "0:00:00.006139",
        "end": "2019-01-31 11:19:50.956646",
        "failed": false,
        "rc": 0,
        "start": "2019-01-31 11:19:50.950507",
        "stderr": "",
        "stderr_lines": [],
        "stdout": "BASH_EXECUTION_STRING='set | grep XXX'\nXXX_BAR=bar_value\nXXX_FOO=foo_value",
        "stdout_lines": [
            "BASH_EXECUTION_STRING='set | grep XXX'",
            "XXX_BAR=bar_value",
            "XXX_FOO=foo_value"
        ]
    }
}