This comes up periodically (and I can't find the last thread I recall where it did), but I'm pretty sure the answer's always "While it's not explicitly spelled out verbatim that hashes are initialized from a LIST of key/value pairs in the order they occur in the list (so the value for the last instance of a given key 'wins'), that's the way Perl has worked for ages and this behavior is tangentially referenced in perldata so don't worry about using it." (The last paragraph under "List Value Constructors" says "Note that just because a hash is initialized in that order doesn't mean that it comes out in that order.")

Update: Knew I'd find it after I'd already posted: Bulk hash population order