I do not see a reason to disallow empty group_by.

What about passing the whole records to the custom (code reference) agregators for some specially named group_how options? Eg.

 ...
 group_how => {
   '&foo' => sub {
    my ( $result, $rec) = @_;
    return $result + $rec->{a} * $rec->{b}
   },
 ...
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producing a new attribute named 'foo' in the resulting records.