I don't have access to a system with XML::XPath installed as of this writing, when I get it I will update if necessary, but I believe the problem is that your xpath query might contain a syntax error. If $CONFIG{teamid} is a string, it needs to be in single quotes for a string equals predicate (the part in brackets). XML::XPath will sometimes fail spectacularly when given a bad query.

If $CONFIG{teamid} is a string the query should look like this:

  $usernodeset = $xpu->find("/users/user[teamid='$CONFIG{teamid}']");
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