sub rindex_substr {
    my ( $string, ) = @_;

    my %pos;
    for my $x ( qw( B C f ) ) {
        $pos{$x} = rindex($string,$x);
    }
    my $replace = ( sort { $pos{$b} <=> $pos{$a} } keys %pos )[0];

    substr( $string, $pos{$replace}, 1 ) = "K";

    return $string;
}