"The net affect of this is that weaken($b[0]) is actually trying to weaken the temporary proxy object rather then the stored reference."

This is actually what i thing (I think that weaken contains a bug) The actual code from Scalar::Util is :

void
weaken(sv)
    SV *sv
    CODE:
        sv_rvweaken(sv);
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I don't know Perl XS, but my guess is that if it is tied, it must fetch the internal value. This is my guess...