If your XML is machine-generated and consistent, a line-by-line regular expression might still get you the results faster. But if your preconditions stretch across multiple lines, I'd stay with XML::LibXML.
From my cursory reading of your XML and your source code, it seems that you are interested in the IntDir and OutDir nodes, as "only these can contain the ReleaseDLL directory" (famous last words here). I'd then make the XPath expression more explicit:
# For the IntDir nodes
# For the OutDir nodes
If you are hell-bent on producing and using one single XPath expression, you can combine the two using the self:: axis. I prefer to avoid such stuff and just keep a list of XPath expressions to run instead:
//PropertyGroup[contains(\@Condition,'$proj')]/*[self::IntDir or self: