> Is it possible to use hash reference as a key in another hash reference?

That's a classic! :)

Hash-keys are always stringified and there is no "direct" way to dereference the stringification.

BUT you are free to keep the original ref somewhere. Either as value in your data-structure or in a separate hash with $ref_of_str{"$key_hash"}=$key_hash. ¹

Thats much safer BTW cause a string doesn't increment the reference count ... i.e. keeping the reference somewhere protects the data from destruction if ref-count reaches 0.

(I hope it's clear now why dereferencing a string will hardly be ever possible)

HTH! :)

update

¹) I rarely come into situations where I really need this trick. I'm sure your data-structure could be rewritten to avoid this, but tl;dr...

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