The following piece of code cancatenates the result of both xpaths.

foreach my $node ($xpc->findnodes('/xpath1 | /xpath2'))
{
    foreach my $col(qw/Id Title Book Publisher Platform/)
    {
        print $xpc->findvalue($chapter_columns{$col}, $node);
    }
}
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I want to first check value of Id in xapth1 and if it is not present, it should look into xpath2.