+ is the unary-+ operator. It does absolutely nothing but evaluate to the same thing its operand does. It even provides the same context as the one in which it's evaluated. 

+5        # Same as `5`
+"abc"    # Same as `"abc"`
+@a       # Same as `@a`, in both scalar and list context

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${ shift } is just a weird undocumented way of writing $shift. I didn't want that. I wanted to use the shift operator. ${ +shift } can't be parsed as a scalar named +shift, so it has the desired effect.

In reply to Re^6: Passing argument by reference (for a scalar) by ikegami
in thread Passing argument by reference (for a scalar) by jmClifford

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