advice 1) perlquote + xpather.pl

advice 2) line up your xpaths by / and study the xpather full paths :) like this

#!/usr/bin/perl --
use strict; use warnings;
use XML::LibXML;

my $dom = XML::LibXML->new(
    qw/ recover 2 /
)->load_xml(
    location => 'pm1082058.xml',
);
for my $matrix ( $dom->findnodes( q{  //matrix/vm[@type='br']  } )) {
    print $matrix->nodePath, "\n";
    print "\n$matrix\n\n";
#~     for my $version ( $matrix->findnodes('./vm/release/@version') )
+{
    for my $version ( $matrix->findnodes('./release/@version') ){
        print $version->nodePath, "\n";
        print "$version\n";
    }
    print "\n\n";
}
__END__
/formation/block/matrix[1]/vm[2]

<vm type="br">
        <release version="7.2.2">
        </release>
      </vm>

/formation/block/matrix[1]/vm[2]/release/@version
 version="7.2.2"


/formation/block/matrix[2]/vm[2]

<vm type="br">
        <release version="7.2.1">
        </release>
      </vm>

/formation/block/matrix[2]/vm[2]/release/@version
 version="7.2.1"
[download]

As you can see vm doesn't have vm children

//matrix/vm[@type='br']
                       ./vm/release
[download]