Yes, this is certainly a bug. Here is what's happening:

Update: This diagnosis is mistaken — see below.

• The way $foo++ is implemented is that the value of $foo is copied to a temporary SV (the op's targ), the original value is then incremented; and finally the top element of the stack is pointed at the targ.
• In this example, the second time FETCH is called, the targ gets overwritten with the new value. But there's already a pointer to it on the stack.

I guess one solution would be to use a temporary mortal instead of the targ.

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