Silly trick:

  my %args = %default, @_;
  foreach my $arg (keys %args) {
    unless (exists $allowed{$arg}) {
      croak("Unrecognized argument '$arg' passed to MemoryStatus.");
    }
  }
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Just put all possible defaults in %default and the user can override any combination they want, and your %allowed check will catch most typos and misunderstandings of the API.