delete $A::{bbb} deletes an entry from the %A:: stash, which means that there is one less reference to the *A::b glob. However, for effeciency the GVSV op (as executed in $A::bbb) doesn't lookup the stash at runtime, but instead at compile time it caches a direct pointer to the glob. These references to the glob ensure that it doesn't go away. An eval shows the difference:

$A::bbb = 3;
delete $A::{bbb};
printf "direct=%d eval=%d\n", $A::bbb, eval q{$A::bbb};

outputs:
direct=3 eval=0
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Dave.