Strong recommendation. Get in the habit of doing the above as:

$guarantee =~ s/OldValue/NewSetting/; # /g?
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rather than playing games with $_. First of all it is more obvious. But secondly you will avoid the problems that I mentioned in Re: Re: $_ haters anonymou - problems that will come back and bite you when you start trying to use features like map and grep. (Which alias each element in a list to $_ and then do something. Well if the do something calls a function like the above...)