my $parser = XML::LibXML->new();
$dom = $parser->parse_file( "OLDFILE" );

for my $matrix ( $dom->findnodes( q{  //version_matrix/vm[@type='br']  } )) {
    my $version = $matrix->findvalue('./release/@version');
    if( $version gt $brver ) { 
        print "discarding version_matrix for $version\n";
        my $version_matrix_node = $matrix->parentNode;
        $version_matrix_node->unbindNode;
    }
}
my $changed = $dom->toString;
my $fh;
open( $fh, '>', "NEWFILE" );
print { $fh }  $changed;