The basic problem is your XPath expression doesn't follow the structure of the document. <mt> is not a child of <md>, there's a <mi> in between. Therefore, something like the following should match all the VAL nodes:

/mdc/md/mi/mt[contains(.,"VAL")]
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The same holds for the <r> elements: their parent is <mv>.

The following works for me:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::LibXML;

my @val;
my $doc = 'XML::LibXML'->load_xml( location => 'file.xml' );
for my $book ($doc->findnodes('/mdc/md/mi/mt[contains(.,"VAL")]')) {
    my $order = 1 + $book->findvalue('count(preceding-sibling::mt)');
    my $rs = $book->findnodes("../mv/r[$order]");
    say join ', ', map $_->textContent, $book, @$rs;
}
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You can get the same logic with XML::XSH2, which is a wrapper around XML::LibXML:

open file.xml ;
for /mdc/md/mi/mt[xsh:match(.,'^VAL[0-9]+')] {
    my $order = 1 + count(preceding-sibling::mt) ;
    echo xsh:join(', ', (.), ../mv/r[$order]) ;
}
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