Very interesting. I can believe that happened.

Still looks to me like a bug.

perl -e '$str = "Hello World\n"; $str =~ s/\r?\n?$/\n/g; print $str;'
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Where did the second return come from?

At the least after matching $ you should not match a zero-width assertion at that point again. IMHO and all that.

I will send that bug report in shortly.