#!/usr/bin/perl -w

use strict;

use XML::GDOME;

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

my $doc = XML::GDOME->createDocFromURI($fname,GDOME_LOAD_SUBSTITUTE_ENTITIES); # 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);
    }
}