The $% just there for obfuscation, it could've been mostly any scalar -- that it's a built-in makes no difference -- as you indicated. However, "recent" optimizations involving shared values between keys makes it not generate the wanted result since the value is constant. For that, different values would be preferable, e.g. $_{$_[0]} = keys %_;.

Not that anyone ever should even think about using this.

ihb