my $letters = shift @ARGV;
my @letters = split //, $letters;
my $pat = qr/^[$letters]+$/;

##</code><code>##

my $wordlist = 'words.en.list'; # assumed English wordlist
my @words = ();
{
  open my $words, '<', $wordlist or die "$wordlist: $! $^E";
  while (<$words>) { push @words, $_ if m/$pat/ }
  close $words;
}

##</code><code>##

sub count_letters ($) {
  my $word = shift;
  my %word = ();
  $word{$_}++ for split //, $word;
  return \%word;
}