have a look at the "look-ahead assertion" in "perldoc perlre"

Your code is just an example to show that normally (when the g switch is set) a regex engine will start looking for it's next match _after_ the last character of the previous match. However, characters matched in a look-ahead assertion don't 'count' as far as the regex engine is concerned, and it will not automatically skip over them when trying to find the next match.

It's an advanced topic, and I really think you'd need to understand how regex engines work to understand this, to which end I'd highly recommend Jeffrey Friedl's Mastering Regular Expressions.

UPDATE: changed wording in second paragraph