In both cases the last part of the regexp is the longest floating string, so is the part that Perl attempts to match first.

I've never seen that be the case and I seriously doubt that it was the case in your test run.

The only thing I've seen the regex engine do with the "longest floating string" is to use only the length it to estimate the offset where it will begin searching for a match (going left-to-right in the string and left-to-right in the regex).