local $_ = 'aaaaabaaaaab';
1 while /
   (
      (?<=a)
      (a)
      (?=[ab])
   )
   (??{ print $^N; '(?=)' })
/gx;
print("\n");

##</code><code>##

local $_ = 'aaaaabaaaaab';
1 while /
   a
   (a)
   [ab]
   (??{ print $^N; '(?=)' })
/gx;
print("\n");