if (exists $hash{$key}) {
  if (ref $hash{$key} and ref $hash{$key} eq 'ARRAY') {
    push @{$hash{$key}}, $new_val;
  }
  else {
    my @letters_array = $hash{$key};
    push @letters_array, $letters;
    my $new_val = [@letters_array];
    $hash{$key} = $new_val;
  }
}
else {
  $hash{$key} = $letters;
}