I have not been able to get SOAP::Lite to output the XML needed to make this work. This is what the SOAP request should look like on the wire:

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
  <s:Body>
    <Invoke xmlns="http://schemas.solarwinds.com/2007/08/informationservice">
      <entity>Orion.Events</entity>
      <verb>Acknowledge</verb>
      <parameters xmlns:a="http://schemas.datacontract.org/2004/07/System.Xml" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
        <a:XmlElement>
          <ArrayOfint xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
            <int>18793</int>
          </ArrayOfint>
        </a:XmlElement>
      </parameters>
    </Invoke>
  </s:Body>
</s:Envelope>

The closest I have been able to get is this, which doesn't work correctly:

<soap:Envelope soap:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" xmlns:msa="http://schemas.microsoft.com/2003/10/Serialization/Arrays" xmlns:msc="http://schemas.microsoft.com/ws/2005/12/wsdl/contract" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:soap12="http://schemas.xmlsoap.org/wsdl/soap12/" xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:swis="http://schemas.solarwinds.com/2007/08/informationservice" xmlns:swpb="http://schemas.solarwinds.com/2007/08/informationservice/propertybag" xmlns:sys="http://schemas.datacontract.org/2004/07/System.Xml" xmlns:wsa="http://schemas.xmlsoap.org/ws/2004/08/addressing" xmlns:wsa10="http://www.w3.org/2005/08/addressing" xmlns:wsam="http://www.w3.org/2007/05/addressing/metadata" xmlns:wsap="http://schemas.xmlsoap.org/ws/2004/08/addressing/policy" xmlns:wsaw="http://www.w3.org/2006/05/addressing/wsdl" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/" xmlns:wsp="http://schemas.xmlsoap.org/ws/2004/09/policy" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" xmlns:wsx="http://schemas.xmlsoap.org/ws/2004/09/mex" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <soap:Body>
    <swis:Invoke>
      <swis:entity xsi:type="xsd:string">Orion.Events</swis:entity>
      <swis:verb xsi:type="xsd:string">Acknowledge</swis:verb>
      <swis:parameters>
        <sys:XmlElement>
          <c-gensym6>
            <int xsi:type="xsd:int">18794</int>
          </c-gensym6>
        </sys:XmlElement>
      </swis:parameters>
    </swis:Invoke>
  </soap:Body>
</soap:Envelope>

Maybe that will be enough information for you to fix it.

Another option would be to switch to the REST endpoint. The JSON-format that it uses it easier to get right. Here's some Perl code for acknowledging an event that way:

require strict;
require REST::Client;
require JSON;
require MIME::Base64;
my $username = 'admin';
my $password = '';
my $hostname = 'latest-stable-builds.swdev.local';
my $swis = REST::Client->new();
$swis->getUseragent()->proxy(['https'], 'http://localhost:8888/');
my $headers = {
  Authorization => 'Basic ' . MIME::Base64::encode_base64($username . ':' . $password),
  'Content-Type' => 'application/json'
};
my $entityName = 'Orion.Events';
my $verbName = 'Acknowledge';
my $uri = "https://$hostname:17778/SolarWinds/InformationService/v3/Json/Invoke/$entityName/$verbName";
my $eventId = 18822;
my @eventIds = ($eventId);
my @args = (\@eventIds);
my $json = JSON->new->allow_nonref->utf8;
my $json_text = $json->encode(\@args);
my $response = $swis->POST($uri, $json_text, $headers);
print $response->responseContent();