something is obvoiusly broken in perlre.html
I'm quoting whole paragraph:
A powerful tool for optimizing such beasts is what is known as an
which does not backtrack (see "look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead <i>might</i> have influenced the following match, see <i>(?>pattern)>>>.</i>" in (?>pattern)>>>). Note also that zero-length look-ahead
(?>pattern)>>>). Note also that zero-length look-ahead
perldoc perlre gives something different.