Palindrome pairs

Given a list of unique words, find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e. words[i] + words[j] is a palindrome.
Input: ["abcd","dcba","lls","s","sssll"]
Output:
(abcd,dcba)
(dcba,abcd)
(s,lls)
(lls,sssll)

Palindrome pairs is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Perl 6

<lang perl6>my @words = <abcd dcba lls s sssll>;

sub is-palindrome (Str $s) { $_ eq .flip given $s.lc.comb(/\w/).join }

for @words.combinations(2) {

   say "({.join: ','})" if .join.&is-palindrome;
   say "({.reverse.join: ','})" if .reverse.join.&is-palindrome;

}</lang>

Output:

(abcd,dcba)
(dcba,abcd)
(s,lls)
(lls,sssll)

Ring

<lang ring>

1. Project : Palindrome pairs
2. Date  : 2018/07/27
3. Author : Gal Zsolt (~ CalmoSoft ~)
4. Email  : <calmosoft@gmail.com>

load "stdlib.ring" pairs = ["abcd","dcba","lls","s","sssll"] for n = 1 to len(pairs)-1

    for m = n + 1 to len(pairs)
         pal1 = pairs[n] + pairs[m]
         pal2 = pairs[m] + pairs[n]
         if palindrome(pal1)
            see "(" + pairs[n] + "," + pairs[m] + ")" + nl
         ok
         if palindrome(pal2)
            see "(" + pairs[m] + "," + pairs[n] + ")" + nl
         ok
    next

next </lang> Output:

(abcd,dcba)
(dcba,abcd)
(s,lls)
(lls,sssll)