Thanks for the tip on YAPE::Regex::Explain.
As kennethk pointed out - reading between the lines - the key is to know that a back slashed special character will not be taken as a word character, and hence will be seen as a word boundary itself. That is why the simple \b does not work.
Armed with a definitive answer as to what is happening, a simple work around can be constructed. In my case the strings I am looking for are actual directory names. I can split on the directory separator.