Is there a possibility to force the sort function to do so? :)

Sure there is! Once you realise what keys is iterating over, it becomes obvious.

The keys of your $hash are 1 and 2, so that's what $a and $b refer to in the sort block.

The final piece of the puzzle is going from there to the right value. It looks to me you want the value of "key1". That gives us:

sort { $hash->{$a}{key1} cmp $hash->{$b}{key1} } keys %$hash
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