sub start_workflow {
    my ($self, $wf_task, $wf_user) = @_;

    my $userAgent = LWP::UserAgent->new(agent => 'perl post');
    my $xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?> 
<SOAP-ENV:Envelope 
    xmlns:SOAP-ENV=\"http://schemas.xmlsoap.org/soap/envelope/\" 
    xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" 
    xmlns:xs=\"http://www.w3.org/2001/XMLSchema\">
    <SOAP-ENV:Body>
        <ns1:SAP_WAPI_START_WORKFLOW xmlns:ns1=\"urn:sap-com:document:sap:rfc:functions\">
            <TASK>$wf_task</TASK> 
            <USER>$wf_user</USER> 
        </ns1:SAP_WAPI_START_WORKFLOW>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>";

    my $response = $userAgent->request( POST $self->{__service},
                                    Content_Type    =>  'text/xml',
                                    Content         =>  $xml,
                                    Authorization   =>  $self->{__authorization}, );
                                    
    return $response->content;
}

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope
    xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/"
    xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP-ENV:Body>
    <ns1:SAP_WAPI_START_WORKFLOW.Response xmlns:ns1="urn:sap-com:document:sap:rfc:functions">
        <NEW_STATUS>
            <STATUS>STARTED</STATUS>
            <STATUSTEXT>in Arbeit</STATUSTEXT>
        </NEW_STATUS>
        <RETURN_CODE>0</RETURN_CODE>
        <WORKITEM_ID>000000005061</WORKITEM_ID>
    </ns1:SAP_WAPI_START_WORKFLOW.Response>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

use SOAP::Lite;
use SOAP::Data qw/name/;

my $soap = new SOAP::Lite(
  uri => 'urn:sap-com:document:sap:rfc:functions',
  proxy => 'http://wherever.it.lives.example'
);

my $som = $soap->SAP_WAPI_START_WORKFLOW(
  name(TASK => $wf_task),
  name(USER => $wf_user)
);
die $som->faultdetail if ($som->fault);

say $som->result;