It was fixed by the following commit, which is a more general fix than just %+:

commit 6f48390ab209d16ee8f795f0a83677c8bd9ac69c
Author: Father Chrysostomos <sprout@cpan.org>
Date:   Wed Jan 4 23:28:54 2012 -0800

    [perl #95548] Returned magical temps are not copied
    
    return and leavesub, for speed, were not copying temp variables wi
+th a
    refcount of 1, which is fine as long as the fact that it was not c
+op-
    ied is not observable.
    
    With magical variables, that *can* be observed, so we have to fore
+go
    the optimisation and copy the variable if it’s magical.
    
    This obviously applies only to rvalue subs.
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Dave.