Yup, I'd probably have gone with a `for` loop also. A bit shorter:

  for set in *_data.csv; do
    [[ -f "${set/data/A}" ]] || echo "${set%_data.csv}"
  done

Edit: though I just write it out like this for formatting on HN. In real life, that would just be a one-liner:

for set in *_data.csv; do [[ -f "${set/data/A}" ]] || echo "${set%_data.csv}"; done