Do you mean something like this?

Win8 Strawberry 5.8.9.5 (32)  Sun 06/05/2022 10:21:09
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

for my $s (
    'AAAAAXXXXXXXAAAXXXXXAXXXXAAAA',  'AXA',
    'XXXXXXXAAAXXXXXAXXXX',  'XXXXX',  'XA',  'AX',  'X',
    'AAAAA',  'YXY',  'A',  '',
    ) {

    (my $t = $s) =~ s{ (?<! [^A]) X+ (?! [^A]) }
                     {@{[ 'a' x ($+[0] - $-[0]) ]}}xmsg;

    print "'$s' \n'$t' \n\n";

    }

^Z
'AAAAAXXXXXXXAAAXXXXXAXXXXAAAA'
'AAAAAaaaaaaaAAAaaaaaAaaaaAAAA'

'AXA'
'AaA'

'XXXXXXXAAAXXXXXAXXXX'
'aaaaaaaAAAaaaaaAaaaa'

'XXXXX'
'aaaaa'

'XA'
'aA'

'AX'
'Aa'

'X'
'a'

'AAAAA'
'AAAAA'

'YXY'
'YXY'

'A'
'A'

''
''
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What is supposed to happen at the begin and end of the string? Please at least give some examples.

the problem statement was substitute all occurences of a character (let's call it 'X') that is inbetween two others