> But I believe your example is also for "all distinct letters", it isn't clear to me how you'd use it for BernieC's templates.

OK taking under assumption, that what the OP meant was to only match strings build from a "template" with an anchored substring having the constraint of (certain?) unique letters, one can easily adapt the above regex solution.

use v5.12;
use warnings;
use Data::Dump;

#ddx
my @words = map { "..X${_}X.." } <{a,b,c,d}{a,b,c,d}{a,b,c,d}>;


my $class = "[abc]";
#$class    = '.';                        #uncomment to match d too


my $len = 3;

for (@words) {
    say "$_"
      if $_ =~ / X                      # start anchor
                 (
                     ($class)
                     (?!
                         .{0,$len}
                         \g{-1}         # relative backreference
                         .*?
                         X
                      )
                 ){$len}
                 X                      # end anchor
               /x;
}
[download]

..XabcX..
..XacbX..
..XbacX..
..XbcaX..
..XcabX..
..XcbaX..
[download]

Hope that's clearer now. :)

update

added part

.*?
X              # end anchor
[download]

update

replaced .*? with .{0,$len}

NB: technically $len-1 would be better, but the end anchor makes this redundant

update

OK I agree that this is a mess. But I'd rather wait for clarifiation from the OP before suggesting better solutions.