my %hash = (
    foo   => 'bar', # Exists, Defined, Positive length, True.
    baz   => '',    # Defined, False, Zero-length, Exists.
    blink => undef, # Exists, Not Defined, Zero-length, False.
    # buzz => ''  # Does not exist, Not defined (because commented out).
);

foreach my $key (qw(foo baz buzz blink)) {
    print "$key ", (exists $hash{$key} ? "exists.\n" : "does not exist.\n");
    print "$key is ", (defined $hash{$key} ? "defined.\n" : "not defined.\n");
    print "$key has a length of ", (defined $hash{$key} ? length($hash{$key}) : "zero because it is not defined"), ".\n";
    print "$key\'s value is ", (defined $hash{$key} ? $hash{$key} : 'undefined'), ".\n";
    print "$key\'s Boolean value is ", ($hash{$key} ? "true.\n" : "false.\n");
    print "\n";
}

##</code><code>##

foo exists.
foo is defined.
foo has a length of 3.
foo's value is bar.
foo's Boolean value is true.

baz exists.
baz is defined.
baz has a length of 0.
baz's value is .
baz's Boolean value is false.

buzz does not exist.
buzz is not defined.
buzz has a length of zero because it is not defined.
buzz's value is undefined.
buzz's Boolean value is false.

blink exists.
blink is not defined.
blink has a length of zero because it is not defined.
blink's value is undefined.
blink's Boolean value is false.