Most probably due to passing the regex as string instead of using qr.

Most certainly due to this. If warnings had been enabled, some Unrecognized escape \s passed through ... messages would have been seen. (And \b is a backspace IIRC.) The problem could also have been avoided by defining the regex as a single-quoted string, but I would not recommend this because I seem to remember that using single-quoted strings in this way still has some corner-case pitfalls. I agree: use of  qr// is best here.