findnodes (note the "s") returns a list of nodes (list context) or a NodeList object (scalar context). You want an actual node. If you want to discard all but the first result, you can use a list assignment:

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   v         v
my ($inc_node) = $xpc->findnodes('/upload/incident[number/text()="INC902531"]');

But your XPath is wrong. It should be

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                                     v                                 v
my ($inc_node) = $xpc->findnodes('/unload/incident[number/text()="INC902550"]');