Juerd,
I was hoping you might reply.

If pre and post are always executed directly after eachother, then what's the use of separating them?

I just threw them in there as I wasn't given a real clear definition of how/when they were supposed to be invoked. I assume they would be separate but only perldeveloper can answer that

It provides a single hook for processing and validation: the code block. The value returned by it determines whether the new value is correct (hence the 1 in the example above), but both $_ and $_1 are aliases for the value that is about to be set, so there's more than enough chance to manipulate the value before it gets assigned.
There is a lot of history from the CB that this post did not capture. After posting it, I did mention in the CB that Attribute::Property might provide aliases in the validation allowing modification if necessary. It is my fault for not looking at the source and not updating my original post accordingly.