But please note the second fOO

  DB<55> p "hello\nfoo" =~ s/o*(?:\n|\z)/O/gmr;
hellOfOO
  DB<56>
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I'm busy right now, but I seem to remember that one could use features for atomic matches...

I'll try later...°

update

°) nah doesn't help, since it's not a backtracking problem.

But please note the second fOO

Yes, good point! I think the main question is what the intent of the regex is. If it's "replace any o's at the end of each line", then the better solution is, as you said, /o+$/, and using o* is the "mistake".

No, the intent is to ensure that the string ends in just one o, no less, no more than one.

In real life, those lines were paths, and I wanted all of then ending in a single slash. For instance:

/foo  --> /foo/
/foo/ --> /foo/
/foo////// --> /foo/
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