sub truth_table {
    my $hash = shift;
    print "       true?   defined?   exists?\n";
    for my $key (qw(foo bar baz bla)) {
        print "  $key  ";
        printf "  %-8d",         $hash->{$key};
        printf "  %-8d", defined $hash->{$key};
        printf "  %-8d", exists  $hash->{$key};
        print "\n";
    }
}

truth_table(
    {
         foo => 1,       # true
         bar => 0,       # false
         baz => undef,   # undefined
         # bla           # doesn't exist
    }
);

__END__

       true?   defined?   exists?
  foo    1         1         1       
  bar    0         1         1       
  baz    0         0         1       
  bla    0         0         0