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