my $min_idx = 5; # EDIT: Den $min_idx eingebaut
my $string = "abcdefABCDEF";

for (my $i = length($string)-1; $i >= $min_idx; $i--) {
        if (substr($string, $i, 1) ~~ ['C', 'B', 'f']) {
                substr($string, $i, 1) = 'K';
                last;
        }
}

say $string;