#!/usr/bin/env perl
use strict;
use warnings;

my %hash = ("fred"   => "flintstone", "dino" => undef, 
            "barney" => "rubble",    "betty" => "rubble",
           );

printf "%10s\t=>%10s\n", "keys", "values";
for ( keys %hash ) {
    printf "%10s\t=>%10s\n", $_, $hash{$_} // '';
}