"Yes, on the one hand, your code does what you think it does" ... but the whole situation smells funny to me, design-wise, because the "uninitialized value" will only acquire a predictable value if this particular piece of code executes. Therefore, at any other point in the execution of the overall program, there are three equally-probable values: undef, $sixth, 3.
If, in the design of your system, $a could legitimately be such a "three-state value," then your code is just as good as any. But, it catches me by surprise. I almost never see it (in any language). Usually, every variable's value – if it is "to always have one" – is pre-initialized at its creation to some default.
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