Most likely, the content of @list_words is not what you think it is. Maybe it contains terminal control codes, or one single large line.

Note that you output @list_words twice, with no delimiter in between, which will make debugging this much harder.

My first attempt at debugging this would be to add some non-whitespace characters around the elements of @list_words while printing them, and cut down @list_words to say 10 elements that you can also share here:

splice @list_words, 10; # inspect only the first 10 elements

use Data::Dumper;
local $Data::Dumper::Useqq = 1; # output strings with whitespace chara
+cters escaped

while (scalar @list_words) {
  test_if_letters_are_included();

  foreach(@list_words) {
    print "list_words:<$_>\n";
    print Dumper $_; # also show the string with whitespace escaped
  }
  my @array = qw(this that the other thing);
  print "@array";
  #say "@list_words";
  say scalar @list_words;
}
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Thanks for the quick reply. I'll give it a try.

I failed to mention that when I run this on my Windows version of Perl it does not have the output problem and prints as expected.

> when I run this on my Windows version of Perl it does not have the output problem and prints as expected.

sounds like you are printing control code characters which are messing with the terminal.

Like a CR without LF might overwrite the same line over and over again.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

I though that at first too. But the subroutine has a prompt for user input after the previous standard output. So the loop stops after every standard out until the array is empty. Both of the print statements are printing after the subroutine so it should be printing same @list_words. Right?

The subroutine asks the user for a string of characters and uses them as a character set in a regex. Any of the words that don't contain any of the characters are removed from the list.

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;
}
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As a general tip for future questions, it's considered good manners here to include a SSCCE.

Without a SSCCE it's hard to tell, but your test_if_letters_are_included subroutine appears to be setting a global variable @list_words. Is that right? In your code, where is @list_words declared? What is its scope?

As a matter of general programming style, a subroutine that sets a global variable is a red flag to me. As for why:

• Action at a distance (wikipedia)
• Side effect (wikipedia)
• Referential transparency (wikipedia)
• Functions with side-effects don't play well with TDD

Another way to write that:

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

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

    # create a character set of input letters
    my $chars = uc join '', $input =~ /[[:alpha:]]/g;

    @list_words = grep /[$chars]/, @list_words;
}
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