A little comment for perl beginner as like me, undestand better this expression.

y/// is Search/Replace operator and the option 's' ignore the dublicated caracteres, 'ddddooo' will be just 'do'.

To finalizy, this script are replacing 'b' to 'a', 'c' to 'b' ... 'k' to 'j'...

Easy as like that ;)