By losing all references to it, which you already do by overwriting all the variables.
A little cleaner and clearer:
use XML::LibXML;
my $parser = XML::LibXML->new();
foreach my $xmlfullname (@xmlfullnamearray) {
my $doc = $parser->parse_file($xmlfullname);
my $node = $doc->findnodes("/root/node");
...
}