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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