I can access the data in the arrayref returned by the query by using @{$months->$x}, but when I try to use @{$months->$x} as the key for the hash, I get nothing - $monthhash{ @{$months->$x} } which I dont understand.

It has been extensively explained to you, but just to stress it: hash keys are strings. Thus you can't use an array as a key, whether it's an actual array or a dereference of an arrayref. You could use "@array" as a key, but generally there's no need to do that and I don't think it's what you have in mind. OTOH you can take a slice of an hash, but in that case that's

@monthhash{ (@{$months->[$x]}) }
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not

$monthhash{ (@{$months->[$x]}) }
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I'm more and more convinced of what I wrote!