Count Zero the issue with combining the elements in such a way is that for each key withn the multiple key set you are producing different arrays. In the op example it is that both keys within the multikey refer to array [1,2,3]. So in your example hash b and c should both hold [1,2,3].

Could we combine the key elements like you suggest but use input processing to determine when either of the key conditions has been met for the required array.

my %hash = (
    {    ab => [1, 2, 3],
         cd => [4, 5, 6],
    },
);


 print $hash{'cd'} if /c|d/;
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the dereferencing syntax may be incorrect here but the example is for the processing of the key.