Another option is XML::GDOME, which, apart from a slightly outdated install process, seems to work well. And has XPath support out of the box. Again another case of the thin perl wrapper around the speedy C implementation. Doco is complete, but it doesnt hold anyones hand - if you know XML reasonably well, you should be fine - if your not familiar with the XML concepts, finding what you need can be difficult.

#!/usr/bin/perl -w

use strict;

use XML::GDOME;

my $fname = '/path/to/your.xml';

my $doc = XML::GDOME->createDocFromURI($fname,GDOME_LOAD_SUBSTITUTE_EN
+TITIES); # or whatever gdome options float your boat

my @nodes = $doc->findnodes('/xpath/to/required/element');

foreach my $node (@nodes) {
    if (needToUpdateAttribute($node)) {
        my $attributeName = getRequiredAttrName(...);
        my $newValue      = getNewAttrValue(...);

        $node->setAttribute($attributeName, $newValue);
    }
}
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