In your example, @_ is a list.

Syntacticaly, it seems like the following assignments are easy to distinguish:
1. list=list
2. list=scalar

I can see reasons why not, maybe a 'scalar' is automatically promoted to a '1-element list'.

If this is not the case, then my suggestion was that in case (2), which doesn't seem to make sense anyway, either a warning will be given or the whole slice be set to the given value.
Case (1) is the same ol' perl.