reverses the reverse

reminds me of a UK West Country phrase

theze bin an gotten whur thee casn't back'n assn't

You would think that it wouldn't be possible but every driver knows that it is.

I have the same trouble with unless. It is very simple but I too often get it wrong. I tend to stick with

... if not exists $options{'r'};
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Not as elegant but it tends to increase my chances :-)