Win8 Strawberry 5.8.9.5 (32)  Sun 06/05/2022  5:38:06
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

my $s = 'AAAAAXXXXXXXAAAXXXXXAXXXXAAAA';
print "'$s' \n";

$s =~ s{ (?: \G (?! \A) | (?<= A)) X (?= X* A) }{a}xmsg;
print "'$s' \n";

^Z
'AAAAAXXXXXXXAAAXXXXXAXXXXAAAA'
'AAAAAaaaaaaaAAAaaaaaAaaaaAAAA'