Don't use a service method when the service is complex. According to wsdl the code could be:

$client = SOAP::Lite->new(
            uri   => 'http://www.webserviceX.NET/',
            proxy => 'http://www.webservicex.net/ConvertSpeed.asmx',
               );

$client->on_action(sub {"http://www.webserviceX.NET/ConvertSpeed"});
$client->autotype(0);
$sheader = SOAP::Header->new('');
$sheader->type(xml => '');
$client->ns('http://www.webserviceX.NET/','web');
$client->envprefix('soapenv');

push @request, SOAP::Data->name(speed => 100)->prefix('web');
push @request, SOAP::Data->name(FromUnit => 'kilometersPerhour')->pref
+ix('web');
push @request, SOAP::Data->name(ToUnit => 'milesPerhour')->prefix('web
+');

$soapResp = $client->ConvertSpeed($sheader,@request);

print $soapResp->result,"\n";
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