#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11146148
use warnings;

my $template = 'abcdefa';

my $regex = '';
my %used;
for my $char ( split //, $template )
  {
  if( exists $used{$char} ) # seen before
    {
    $regex .= "\\$used{$char}";
    }
  else # new different character
    {
    $regex .= join '', map("(?!\\$_)", values %used), "(.)";
    $used{$char} = 1 + keys %used;
    }
  }

my $qr = qr/^$regex$/; # the real regex

use Data::Dump 'dd'; dd $qr;

@ARGV = '/usr/share/dict/words';
/$qr/ and print, exit while <>; # show first match