use strict;
use warnings;

my $str = 'XAXAAAAXXXXXXXAAXXXXXAXXXXAAAAX';
my $before = $str;

$str =~ s/(?<=A)(X+)(?=A)/'a' x length $1/ge;
print "$before\n";
print "$str\n";

##</code><code>##

XAXAAAAXXXXXXXAAXXXXXAXXXXAAAAX
XAaAAAAaaaaaaaAAaaaaaAaaaaAAAAX