In PL/SQL, valid variable names must start with a letter and can include letters, numbers, and special characters like underscores, dollar signs, and hash signs, but they cannot begin with a number and should not contain spaces or apostrophes. The variable 'printer_name#' is valid because it starts with a letter and includes an allowable special character. 'leap$year' is valid as it starts with a letter and includes a permissible special character. 'v_fname' is valid as it follows the standard naming convention without starting with a number or including disallowed characters.