Curious question: Exactly how big impact does those variables actually have? I never use them, since it's been hammered into me that I shouldn't because of performance issues. This makes sense, and one rarely needs them anyways. But I'm just a bit curious on what size performance hit are we talking about here? Microseconds, seconds, minutes?

Also, perlre says: once you've used them once, use them at will, because you've already paid the price. If I read that right, it means that the performance hit is only triggered once, the first time one uses them.

I could make a point here about coding for simplicity instead of (unnecessary) performance, but mainly, I am just curious. Is this advice something that is given as a knee-jerk response, and because we all want to have great performance, or is the impact really so large that it matters in usual cases?

All that aside, I do agree that it is a bad idea to use them in any module that might be used by someone else - there is no telling what the performance considerations might be for that script. If you are using English, performance is probably not what you are looking for. But there are probably other examples.

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