Did you try changing

my $query = "/nvd/entry[@id = $cve_id]";
[download]

to:

my $query = "/nvd/entry[@id = '$cve_id']";
[download]

And regarding a better parser it depends on how much information you want to read, XML::LibXML use a DOM Parser which builds a tree model for the document and then you can access that tree, the other standard parsing model is SAX which is based on events, and that is the fastest approach. Of course SAX parsers are much more complicated but in my experience it resulted on an improvement of 500%.