Since you have a template and talk about generating a pattern,

my %captures;
my $pat =
   join "",
      map {
         if ( $captures{ $_ } ) {
            "\\$captures{$_}"
         }  else {
            my $capture = 1 + keys( %captures );
            $captures{ $_ } = $capture;
            if ( $capture == 1 ) {
               "(.)"
            } else {
               "(?!".(
                  join "|",
                     map { "\\$_" }
                        1 .. $capture-1
               ).")(.)"
            }
         }
      }
         split //, $template;

my $re = qr/^$pat\z/s;
[download]

For

my $template = "abcdefa";
[download]

this generates

^
(.)
(?!\1)(.)
(?!\1|\2)(.)
(?!\1|\2|\3)(.)
(?!\1|\2|\3|\4)(.)
(?!\1|\2|\3|\4|\5)(.)
\1
\z
[download]

(Line breaks added for readability.)