I need to change some xml data, and I'm stuck trying to identify the node to modify, so hoping someone can dispel my ignorance.

This is a sample of the xml data file ('n4000-small.xml'); the real data has many matrix elements, not just two, and has siblings of the vm elements:

<formation name="stoneridge" version="1.6">
  <block> 
    <matrix>
      <vm type="sns">
        <release version="8.5.e">
          <supported>
            <fixed>123.1106</fixed>
          </supported>
        </release>
      </vm>
      <vm type="br">
        <release version="7.2.2">
        </release>
      </vm>
    </matrix>
    <matrix>
      <vm type="sns">
        <release version="4.1.e">
          <supported>
            <min>124.1306</min>
            <max>124.1500</max>
          </supported>
        </release>
      </vm>
      <vm type="br">
        <release version="7.2.1">
        </release>
      </vm>
    </matrix>
  </block>
</formation>
[download]

I need to locate the 'matrix' node which has a 'vm' node of 'type="br"' with a specific 'release version' attribute value, then change that matrix's 'vm type="sns"' values (more on that below). I can't figure out what's wrong with my code:

use strict;
use warnings;
use XML::LibXML;

my $file = 'n4000-small.xml';
my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($file);

# THIS WORKS - but it's finding the vm node not the matrix node
foreach my $vm ($doc->findnodes('//vm[@type=\'br\']')) {
    print  "br  ", $vm->findvalue("./release/\@version"), "\n";
    if ( '7.2.1' eq $vm->findvalue("./release/\@version")) {
        print "found 7.2.2\n"
    }
}

# this also works, finding the "br" vm with the right version, but I t
+hink I need the matrix, to identify the "sns" vm

my @wanted_vm  = ($doc->findnodes('//vm[@type=\'br\' and ./release/@ve
+rsion=\'7.2.1\']'));
if( @wanted_vm ) {
    print "found br ", $wanted_vm[0]->findvalue('./release/@version'),
+ "\n";
}

# this gets the 'br' matrices, but why doesn't it get the vm attribute
+s?

foreach my $matrix ($doc->findnodes( '//matrix/vm[@type=\'br\']' )) {
    if( $matrix ) {
        #print "matrix:", $matrix->toString(), "\n";
        print "vm release version:", $matrix->findvalue('./vm/release/
+@version'), "\n";
        my $vm_type = $matrix->findvalue('./vm/@type');
        print "vm type:'" . $vm_type .  "'" . "\n";
    }
}
[download]

Given that 'matrix' is parent of 'vm', can some kind, wiser monk please say why xpath './release/@version' of a vm node works, but './vm/release/@version' xpath from a matrix node does not? (I'm not certain this is the right terminology, but hope it's understandable.)

I haven't got as far as changing the "sns" vm's data yet. There will be two things to do: (i) change the value of the sns vm's 'release version' attribute; and (ii) either change the '//supported/fixed' element's text, or if there are '//supported/max' and '//supported/min' elements they must be removed and replaced by a '//supported/fixed' element with given text. Any gestures in those directions (especially on how to add the 'fixed' element) will also be very helpful and much appreciated.