use 5.010;
use warnings;

my $original = lc "Ceterum censeo Carthaginem esse delendam";

my $cyphered = $original;
$cyphered =~ tr/a-z/k-za-j/;
say $cyphered;

my $decyphered = $cyphered;
$decyphered =~ tr/k-za-j/a-z/;
say $decyphered;