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

##</code><code>##

$string =~ s/(.*)b/$1x/;

##</code><code>##

$string =~ m/.*(?=b)/g and $string =~ s/\Gb/x/;

##</code><code>##

substr($string, rindex($string, 'b'), 1) = 'x';