Mini-Tutorial: Scalar vs List Assignment Operator covers this topic in detail.

So it seems like the value of a list assignment in scalar context is the scalar-context value of the list on the right-hand side of the assignment

No.

>perl -E"say scalar( (4,5,6) )"
6

>perl -E"say scalar( ($x,$y)=(4,5,6) )"
3
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List assignment in scalar context evaluates to the number of values to which its RHS evaluates.

the value of the list assignment in list context is the list-context value of list on the left-hand side of the assignment.

Yes.

List assignment in list context evaluates to the values to which its LHS evaluates (as lvalues).