To apply this code to the data structure as given by the op, this gives:

    @sorted = sort { $a->{key2} cmp $b->{key2} ||
                     $a->{num1} <=> $b->{num1} ||
                     $a->{key3} cmp $b->{key3} ||
                   } @data;
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Note that to sort descending for one column, just swap $a and $b in the comparison expression, for that column.