gistfile1.txt

class fooclass (
  $foo = "using default"
) {
  warning("foo = [$foo]")
}

class { 'fooclass': foo => hiera('does not exist', undef) }

# This prints Warning: foo = []

class fooclass (
  $foo = "using default"
) {
  warning("foo = [$foo]")
}

class { 'fooclass': foo => undef }
# This prints Warning: foo = [using default]