Write a perl program that prompts a user for the letter 'a' or 'b'. If the user gives the program the letter 'a', then the program should print out the phrase "a is for apple." If the user provides the program the letter 'b', then the program should print out the phrase "b is for banana." If the user does not provide the letter 'a' or 'b' then the program should print an error message.

In a perl program