c:\@Work\Perl\monks>perl -wMstrict -le
"use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 for my $ar_vector (
   [ [], ],  [ [], '' ],  [ [], '', '' ],  [ [], '', '', '' ],
   [ [ 'A' ], 'A' ],  [ [], qw(A A) ],  [ [], qw(AA) ],  [ [], qw(A A A)  ],
   [ [ qw(A B) ], qw(A B) ],  [ [ qw(A B C) ], qw(A B C) ],
   [ [ qw(A B C D E F G H) ], 'HGFE', 'ABCD' ],
   [ [ qw(A B C D E F G H I) ], 'GFED', 'HI', 'ABC' ],
   [ [], 'TTTTTTT', 'TTTTTTTTT' ],
   [ [ 'A' ], 'TTTATTTT', 'TTTTTTT' ],
   [ [ qw(A B) ], 'TBTTTTT', 'TTTATTT' ],
   [ [ qw(A B) ], 'TTTTTTA', 'TTTTTTB' ],
   ) {
   my ($ar_expected_singletons, @strings) = @$ar_vector;
   ;;
   my $strings_cmnt = qq{@{[ map qq{'$_'}, @strings ]}};
   ;;
   my @got_singletons = singletons(@strings);
   is_deeply \@got_singletons, $ar_expected_singletons,
     qq{($strings_cmnt) singleton(s): (@$ar_expected_singletons)};
   }
 ;;
 done_testing;
 ;;
 exit;
 ;;
 sub singletons {
   my %freq;
   for (@_) {
     for my $o (0 .. length($_)-1) { ++$freq{substr $_, $o, 1} }
     };
   return sort grep $freq{$_} == 1, keys %freq;
   }
"
ok 1 - () singleton(s): ()
ok 2 - ('') singleton(s): ()
ok 3 - ('' '') singleton(s): ()
ok 4 - ('' '' '') singleton(s): ()
ok 5 - ('A') singleton(s): (A)
ok 6 - ('A' 'A') singleton(s): ()
ok 7 - ('AA') singleton(s): ()
ok 8 - ('A' 'A' 'A') singleton(s): ()
ok 9 - ('A' 'B') singleton(s): (A B)
ok 10 - ('A' 'B' 'C') singleton(s): (A B C)
ok 11 - ('HGFE' 'ABCD') singleton(s): (A B C D E F G H)
ok 12 - ('GFED' 'HI' 'ABC') singleton(s): (A B C D E F G H I)
ok 13 - ('TTTTTTT' 'TTTTTTTTT') singleton(s): ()
ok 14 - ('TTTATTTT' 'TTTTTTT') singleton(s): (A)
ok 15 - ('TBTTTTT' 'TTTATTT') singleton(s): (A B)
ok 16 - ('TTTTTTA' 'TTTTTTB') singleton(s): (A B)
1..16
ok 17 - no warnings
1..17