The regex asks for a single lowercase letter from the Latin alphabet. If you want to have at least one of those and do not care for an upper bound you need to use the + quantifier: ~r/^[a-z]+$/.
Letters and numbers is broad… I consider ä a letter, do you? What’s about Chinese scripts? Or kyrillic? Greek? Persian numbers? Do you consider those as valid?
If you restrict to Arabic numerals and Latin letters in upper and lowercase it’s something like this: