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

if (exists $hash{'foo'}) {
  print "The key 'foo' exists.\n";
} else {
  print "The key 'foo' does not exist.\n";
}

if (defined $hash{'foo'}) {
  print "The key 'foo' is defined.\n";
} else {
  print "The key 'foo' is undefined.\n";
}

if ($hash{'foo'}) {
  print "The key 'foo' is true.\n";
} else {
  print "The key 'foo' is false.\n";
}