package MyApp::SomeObject;
  use constant DEBUG => $ENV{MYAPP_DEBUG};
  use Moo;
  has foo => (
    is => 'ro',
    ( DEBUG ? (
      isa => sub {
        my $value = shift;
        if ($value !~ /^[0-9]+$/) {
          die "Not an integer: $value";
        }
      }
    ) : () ),
  );