You can't because this is a bizarre non-sensical data structure. The part [ key3 => 'Hello' ] is a hash key. It will be coerced to its string representation, e.g. ARRAY(0x759de8). When dumped again, we see something like: 
$hash = {
    key1 => {
        key2 => {
            "ARRAY(0x759de8)" => {
                key4 => 'World'
            }
        }
    }
};

[download]
I suggest you think again about how you construct this structure, there must be a mistake somewhere.

In reply to Re: Access Complex Hash by Anonymous Monk
in thread Access Complex Hash by perl@1983

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