my $instances = $ec2->run_instances(
    ImageId  => $ami_id,
    MinCount => 1,
    MaxCount => 1,
    KeyName  => $key,
    (defined $user_data ?
       (UserData => MIME::Base64::encode_base64($user_data)) : ()),
    (defined $instance_type ?
       (InstanceType => $instance_type) : ()),
  #...
  ) or die "$ec2->{error}";