My approach (not knowing that much about XPath) would be to walk up the tree/parent axis and build the path to the current node from that (I'm thinking in XML::LibXML syntax, sorry):

sub get_path {
    my ($node) = @_;
    my $path = '';;
    my $p = $node;
    while (defined my $p = $p->parentNode) {
        $path = $p->nodeName . "/" . $path
    };
};
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... but that method breaks down as soon as you have to nodes with the same tag. You will have to add the index of the current node then, which I don't know how to get.

This whole method assumes that XML::Parser also has some of the methods the DOM/XML::LibXML has...