I'm now at a box which runs Perl and can test:

\G doesn't help, forgot about the limitations!¹

But the pos approach works:

  DB<129> $_='12aaa67aaaa23'
 => "12aaa67aaaa23"

  DB<130> print pos($_) while ($_=~/(a\Ga)/g)


  DB<131> print pos($_),"\n" and pos($_)=pos($_)-1 while ($_=~/(aa)/g)
4
5
9
10
11
[download]

HTH! :)

¹) perlretut : the \G anchor is only fully supported when used to anchor to the start of the pattern.