Your code fails because it assigns a single scalar value to the lvalue list produced from a hash an array slice. Only one of the elements in the slice will be populated. To populate each element, you can use the x operator in list context.

my @a;
@a[2..5] = (2)x4;
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...or, using map

@a[2..5] = map 2, 2..5;
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...or, using a foreach loop...

$a[$_] = 2 for 2..5;
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There are many other ways to do it as well. Just pick the one that is easiest to read.