Thinking about this one more time ... in case the question actually is not "is the XML valid", but rather "is the XML already complete, assuming it's being uploaded from a valid source" then the XML parser modules based solutions are an overkill. For a quick check with minimal memory footprint something like this may be better:

sub XMLisComplete {
    my $file = shift();
    open my $IN, '<', $file or return; # if I can't open it, it's prob
+ably locked. Therefore it's not complete.
    my $main_tag;
    read $IN, $main_tag, 1024;
    if ($main_tag =~ m{<(\w+)}) {
        $main_tag = $1;
    } else {
        return; # there's not even the opening tag!
    }
    seek $IN, 2, -100;
    my $end = do {local $/; <$IN>};
    close $IN;
    if ($end =~ m{</$main_tag>\s*$}s) {
        return 1;
    } else {
        return;
    }
}
[download]

It's most likely not 100% standard proof (e.g. I bet \w+ doesn't match all allowed tag names), but it works for me to test whether I can already start parsing the uploaded file or whether to wait a bit more. The actual parsing would of course be best left to a module.