> AAAAAXXXXXXXAAAXXXXXAXXXXAAAA

> becomes:

> AAAAAAAAAAAAAAAAAAAAAAAAAAAAA

replacing "X" -> "a" for better demo

use v5.12.0;
use warnings;

my $in  = 'AAAAAXXXXXXXAAAXXXXXAXXXXAAAA';

my $out =
  $in =~ s
  [ (?<=A) X+ (?=A) ]
  [ $& =~ tr/X/a/r  ]gexr;

say $in;
say $out;
[download]

AAAAAXXXXXXXAAAXXXXXAXXXXAAAA
AAAAAaaaaaaaAAAaaaaaAaaaaAAAA
[download]

update

simplified $1 to $&