Aliases would be less expensive once created. However, using aliases introduces more magic (especially if Data::Alias is involved) and creates more action at a distance (making the code less clear and harder to debug).

Actually, even simpler would be to use neither. The problem only exists because the addition of the scalars to the data structure is done at the top of the code.

my ($var1, $var2, ...);
...
push @array, [ $var1, $var2, ... ];
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The downside is that you might have to guard against exceptions and returns.