my %hash = ();
$hash{'bar'} = undef;

##</code><code>##

$hash{'bar'} = '';

##</code><code>##

$hash{'bar'} = undef;

if(defined $hash{'bar'}){print "false"}
if($hash{'bar'}){print "false"}

##</code><code>##

$hash{'bar'} = '0';
if(defined $hash{'bar'}){print "true")
if($hash{'bar'}){print "false")