You have the Moose::Meta::Attribute object already in $build_attribute in the outer loop, so you can do your test simply on that one:

$result = $build_attribute->type_constraint->is_a_type_of('ArrayRef') 
+? 'it's an ArrayRef' :'not an ArrayRef';
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I just tried this in one of my Moose classes and it works for me that way.