sub test_if_letters_are_included{
    
    # Ask for input
    say 'Which letters would you like to use?:';

    # Capture user input
    my $input = <STDIN>;

    # Remove the trailing newline
    chomp $input;

    # Change the input to upper case.
    $input = uc $input;

    # create a character set for the regex
    my $regex ="[$input]";
    
    my @temp_list;

    for (@list_words) {  
      if ($_ =~ /$regex/g) {
        push @temp_list, $_;
      }
    }
    @list_words = @temp_list;
}