How do I set read-only variables in BUILD?

Why not just make a private writer, like this

has 'foo' => (
    is     => 'ro',
    isa    => 'Something',
    writer => '_set_foo',
    lazy   => 1,
    build  => '_build_foo',
);
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Which is called first, BUILD or the attribute_builders?

The entire object instance is created and finalized before BUILD is called. When it gets to BUILD you can be sure you have a complete object (minus any non-initialized lazy attrs, etc).