The $tree - is it an array? a scalar? a variable?

ref can answer that for you:

print ref($tree), "\n";
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outputs:

ARRAY
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Or, if you read the free manual (XML::Parser), it says:

For elements, the content is an array reference.
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Or, looking at the Dumper output:

$VAR1 = [
          'list',
          [
            {
              'name' => 'name list'
            },
            0,
            '
        ',
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the square bracket signifies an array ref.

how do I get access to it?

perlreftut is a good place to begin figuring out how to access Perl data structures such as this.