Cool. Accessing $a[$i] isn't enough to vivify (otherwise your defined test would do it).

So we don't actually need the defined test in your map and can get away with:

# This *doesn't* vivify $a[3,4,5]
@b = map { $a[$_] } (0..5);
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Thanks.

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.