Using /g in the scalar context of an if statement is probably superfluous.

s/superfluous/a mistake/. //g in a scalar context can make your match fail every so often. I've seen this type of bug reported here several times.

This particular code, in isolation, probably wouldn't notice the bug (perhaps that is what you meant) since $str appears to be getting updated at least once per regex application. But the use of //g in a scalar context is something that should be vigorously avoided unless you need that advanced magic and understand it.

I've seen this mistake enough that I'd support a pragma that makes this illegal unless you ask for it. But, in practice, I don't see how to make such useful.