$userInput = <STDIN>;
chomp $userInput;
$userInput = uc $userInput; 

##</code><code>##

my %pbactions = (
    L    => \&list_phone_book,
    LIST => \&list_phone_book,
    A    => \&add_phone_book,
    ...
);

# Later on~

defined $pbactions{$userInput} ? $pbactions{$userInput}->()
                               : usage();