The point of the defined wasn't to prevent autovivification, but to implement the grep you had in your original code. The following two snippets are equivalent.

@b = map { defined($a[$_]) ? ($a[$_]) : () } 0..5;
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@b = grep { defined } map { $a[$_] } 0..5;
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What you suggested (@b = map { $a[$_] } 0..5;) is *not* equivalent.