To call a subref, use $subref->(args).

Note, however, that the keys of a hash are always plain strings; they can't be references. If you try to use a ref, you'll end up getting the stringified version, which can never be reconstructed into a real reference. (Well, that isn't strictly true, but is close enough.)

So what you probably want to say is more like:

foreach (values %FunctionHash) {
  $return = $_->($data);
}
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