use List::MoreUtils 'firstidx';
use List::MoreUtils 'firstidx';
my @letters = a..z;

sub get_next_index {
    my $index = shift;
    my $max   = scalar(@letters);

    my $i = 0;
    while($i != 2) {
        $i++;
        if ($index+2 > $max) {
            $index = -1;
        }
        $index++;
    }

    return $index;
}

sub translate {
    my $str = shift;
    my @string = split undef, $str;
    my @final_string;
    for my $char (@string) {
        if ($char =~ /\s+/) {
            push @final_string, ' ';
            next;
        }
        my $index = -1;
        $index = firstidx { $_ eq $char } @letters;
        if ($index) {
            push @final_string,
                $letters[get_next_index($index)];
        }
    }

    return join(undef, @final_string);
}

my $get_string = translate("fcjjm umpjb bgb wms ilmu rfc qsl gq wcjjmu");
print "$get_string\n";