In perl, a tie on arrays works by creating a temporary proxy object when you do an array subscript. When you later try to read or write to this object, it triggers the call to FETCH or STORE as appropriate. It is necessary to do it like that to allow code like this to Do The Right Thing:

sub p1 { print $_[0]} p1($tied_array[0]); # calls FETCH
sub p2 { $_[0] = 0 }  p2($tied_array[0]); # calls STORE
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The net affect of this is that weaken($b[0]) is actually trying to weaken the temporary proxy object rather then the stored reference. (At least that's what I'm speculating; I've got a cold and it's too much effort to check right now)

Dave.