DB<130> @a = ("abc","aab","abb","aba") 

  DB<131> for (@a) { say "$_ : $1,$2" if /^(.)(?!.*\1)(.)(?!.*\2)/ }
abc : a,b

  DB<132>

##</code><code>##

  DB<142> @a = ("abc","aab","abb", "aba","abcd","abca" )

  DB<143> for (@a) { say "$_" if /^ ( (.) (?!.*\g-1) )+ $/x }
abc
abcd

  DB<144>