If maintaining a one-to-one relationship of %helmholtz_to_midi keys to %midi_to_helmholtz keys is an important requirement, then, yes, you have to do something kludgy like use a compound identifier. For example:

    $helmholtz_to_midi{'a♯¹/b♭¹'}{midi_note_number} = 70;

and

    $midi_to_helmholtz{'70'}{helmholtz_note_name} = 'a♯¹/b♭¹';

But the inescapable musical fact remains:  there isn't a one-to-one relationship of MIDI note numbers to Helmholtz note names. There's a one-to-many relationship, just as there's a one-to-many relationship of pitches (440 Hz) to enharmonic notes (g𝄪¹, a¹, b𝄫¹).