use strict;
use warnings;

my $chars = 'ABC---';

my %seen;
my @letters = grep /[a-z]/i, split '', $chars;
my $regex = join '.*', @letters;
my $string = ( '{' . ( join ',', split '', $chars ) . '}' ) x length $chars;

print "$_\n"
  for grep { /$regex/ and !$seen{$_}++ and @{ [/[a-z]/ig] } == @letters }
  glob $string;