for (qw[ ab abb abbb aabb aabbb aabbbb a_abb a_abbb ]) {
   m[
      (?<!a)
      (?>
         (a+) (?{ length($^N) })
         (b+)
      )
      (?(?{ length($^N)-$^R != 1 })(?!))
   ]x
      and print("$_\n");
}

##</code><code>##

abb
aabbb
a_abb