Somewhere between a bug or feature:

[parisse]

There is a indeed a difference between b|x=2 and b+x|x=2. The first argument of | is not evaled (otherwise the value of any identifier inside this expression would be replaced, including x in this example if x was assigned), but there is an exception: if the first argument is an identifier, it is replaced by its value (but the value is not evaled). The reason is that you have no reason to enter x|x=2 (2 is shorter) or b|x=2 (b would be shorter if it was not replaced).