In my rules, do I refer to this tag as "ns2:Height" or as "Height" somehow accounting for the namespace?

You can assign namespace prefixes using the namespaces option, for example namespaces => { 'http://www.example.com/foo' => 'ns2' }, and then use the ns2: prefix in your rules. Or, you can use the same option to map different namespaces to the same prefix, if that is acceptable in your application (!), in the way I showed below (in that case the empty prefix).

Can I have rules that just deal with Height, Length, Width, Weight, etc?

Yes, you can specify multiple tag names for a rule with |. You can also use regexes to specify rules, as in '/foo/' => ... (although I don't think the two can be combined). Here's an example:

use warnings;
use strict;
use XML::Rules;
use Data::Dump;

my $parser = XML::Rules->new(
    stripspaces => 3|4, warnoverwrite => 1,
    namespaces => {
        'http://www.example.com/foo' => '',
        'http://www.example.com/bar' => '',
        '*' => 'die' }, # don't allow other namespaces
    rules => [
        'Height|Length|Width|Weight' =>
            sub { lc $_[0] => $_[1]->{_content} },
        'root' => 'pass',
        _default => sub { die "Unknown tag $_[0]" },
    ] );

dd $parser->parse(<<'END_XML');
<root xmlns="http://www.example.com/bar"
    xmlns:foo="http://www.example.com/foo" >
    <foo:Height Units="inches">1.00</foo:Height>
    <foo:Length Units="inches">1.00</foo:Length>
    <foo:Width Units="inches">1.00</foo:Width>
    <foo:Weight Units="pounds">0.01</foo:Weight>
</root>
END_XML

__END__

{ height => "1.00", length => "1.00",
    weight => 0.01, width => "1.00" }
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In this code, I've made the _default rule be to die, that way I can make sure during development that I haven't overlooked a tag in the input XML. But you can of course also specify any _default rule you like.

By the way, since these questions are going a bit off topic from the original post, it might be best to start a new thread on your questions about XML::Rules, showing your code, enough example input to reproduce, etc. (Short, Self-Contained, Correct Example).

Thank you very much; that is indeed what I needed (and was missing). That example is right on point!

Agreed on the separate issue that would have been better for the XML::Rules. I was really trying to solve the same issue (in my head) so this is, for me, the same ultimate issue on how to get my data out of the XML. The XML::Simple representation of the data structure made the initial question about the array the starting point.

I will keep your recommendations in mind for the future.