Another way to interpret it is that Perl uses the same mechanism C uses to perform this op, so however C behaves on your system, Perl should behave the same...

Perl doesn't use the same mechanism as C, however. Perl uses an optree, which (compiled) C doesn't do. (A C compiler may use a similar tree structure for optimizations, but it doesn't guarantee the order of operations for similar reasons, not because it uses the same mechanism.)