v is not available in projection
WITH data AS ([{"cc":"us", "app_id":"123"},{"cc":"us", "app_id":"456"}])
SELECT RAW "meta:" || d2.app_uuid || ":" || FIRST v1.cc FOR v1 IN data WHEN "app:a2t:" || v1.app_id == META(d2).id END
FROM `bucket` AS d2 USE KEYS ARRAY "app:a2t:" || v.app_id FOR v IN data END
OR
WITH data AS ([{"cc":"us", "app_id":"123"},{"cc":"us", "app_id":"456"}])
SELECT RAW "meta:" || d2.app_uuid || ":" || d.cc
FROM data AS d
JOIN `bucket` AS d2 ON KEYS "app:a2t:" || d.app_id ;