Your second example isn't doing what you intend. The array outside of the sub isn't modified. The first line of prepData2 sets $refData to the incoming reference. The second replaces that reference with a different one, leaving the contents of the first reference unchanged. You'd want something more like:

sub prepData2 {
    my $refData = shift;
    @$refData = map { log $_ } 1 .. 100;
}
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With that change, the benchmark changes somewhat, making it almost identical to array_ref_out1 in my tests.