#!/usr/bin/perl -w

use strict;

my $string = 'aabbcc';

$string = reverse $string;
$string =~ s/b/X/;
$string = reverse $string;

print $string, "\n";

$string = 'aabbcc';

$string =~ s/b([^b]*)$/X$1/;

print $string, "\n";

$string = 'aabbcc';

my $idx;
substr( $string, $idx, 1, 'X') if (-1 != ( $idx = rindex( $string, 'b'
+ ) ));


print $string, "\n";
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Update:

A zero-width positive look-ahead assertion:

$string =~ s/b(?=[^b]*$)/X/;
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