I think I disagree with your suggestion that $scalar = x() should imply anything about how that function behaves in list context. You get the same issue without wantarray. For example,

my @arr = qw/1 2 3 4 5 6 7 8 9/;
sub x {
    return @arr;
}
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Now, $scalar = x() returns the length of the array. @list = x() returns the actual array. There's no wantarray in sight, but we still get different behavior in different contexts.

Granted, wantarray can be used to generate hard-to-understand software. Then again, so can almost any feature of every language out there.