gistfile1.txt

$ cat test.yaml 
---
- hosts: localhost
  gather_facts: false

  vars:
    cache_mgmt:
      dev_web:
        backend_name: www_http_be
        backend_hosts: dev_www
      dit_web:
        backend_name: www_http_be
        backend_hosts: dit_www

  tasks:
    - set_fact:
        backend_name: "{{ cache_mgmt[configs_target].backend_name }}"

    - debug:
        var: backend_name
       
       
$ ansible-playbook test.yaml -e '{"configs_target":"dev_web"}'

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

TASK [set_fact] ***************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [debug] ******************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "backend_name": "www_http_be"
}

PLAY RECAP ********************************************************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0