I didn't say COW couldn't happen here; I said I didn't think COW would help here.

I'm not sure what you think is being assigned from, but it is indeed a scalar, and thus COW could apply. However, if it does, it might simply replace another pre-existing optimization. This optimization allows a scalar to steal the string buffer of the scalar being assigned to it (i.e. copy the pointer instead of copying the whole buffer). This is only done if the scalar being assigned is a "temp" about to be destroyed (I'm sure what the exact condition is.)