# $str contains the characters to encode
my @chars = split //, $str;
for (@chars) {
    if( $_ eq 'a' ) { $_ = 'n' }
    if( $_ eq 'b' ) { $_ = 'o' }
    if( $_ eq 'c' ) { $_ = 'p' }
    # And so on. I think you all see where this is going
}