Assign To A Function Result

I'm trying to create an ActiveXObject from the passed in parameter. I'm getting the following error 'Can Not assign to a function result.' How do I get around this?

<script lanaguage="javascript">
var PageXml = '<Doc><Node1/><Node2/></Doc>'
function LoadXmlData(Source)
{
var XmlSrc = eval(Source);

eval('Source') = new ActiveXObject("Microsoft.XMLDOM");//ERROR: Can Not assign to a function result.

Source.async = false;
Source.loadXML(XmlSrc);
}
</script>
<button id="2" onclick="LoadXmlData('PageXml');">test</button>