First, do you realize
(??{ print $^N })
is causing the regexp to try to match "1", the return value of print? You should probably use something that's guaranteed not to match like
(??{ print("$^N\n"); '(?!)' })
or something that's guaranteed to match like
(??{ print("$^N\n"); '(?=)' })
(depending on your intention) instead of relying on the return value of print and the presence/absence of the return value of print in the string to match.

You use /g, so I assume you'll call the regexp more than once in a scalar context, or in a list context.

So compare

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

with

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

The first outputs "aaaaaaaa", which the second outputs "aaaa".

(?=...) is useful when followed by something which isn't zero-width, when /g is used and when used in substititutions.

(?<=...) is useful when /g is used and when used in substititutions.