You're assuming code exists in isolation. I can't think of any code that needs the number of elements in a hash that would be unclear no matter how you spelled
0+keys(%h)
But that doesn't mean they're all equal.
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The 0+ points out that something special is happening, whereas scalar() just looks like a function call. A function call that doesn't exist. There's not even an op for it.
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The 0+ points out that a number is going to be returned, confirming what one might think keys is doing.
If you used scalar, they might not know exactly what it did but they'd have a direct pointer to documentation that would explain exactly what the intent of the code was.
I think they'd be better off reading the docs for keys, not scalar. The former lists the two possible return values for keys. The understanding of context will follow.
