XML::twig is probably your best bet for doing this sort of stuff with XML or XHTML.

If you have trouble getting going with XML::twig tell us in a little more detail just what you want to be able to do. My guess is that you actually want to extract some of that data, but you don't actually say that. The following may get you started:

use strict;
use warnings;
use XML::Twig;

my $t= XML::Twig->new (twig_roots => {'Data' => \&extract});
$t->parse (do {local $/; (<DATA>)});


sub extract 
{
my ($t, $data) = @_;
$data->print;
print "\n";
}

__DATA__
<Row AutoFitHeight="0">
<Cell><Data ss:Type="String">DATA1</Data></Cell>
<Cell><Data ss:Type="String">DATA2</Data></Cell>
<Cell><Data ss:Type="String">DATA3</Data></Cell>
<Cell><Data ss:Type="String">DATA4</Data></Cell>
<Cell><Data ss:Type="String">DATA5</Data></Cell>
<Cell><Data ss:Type="String">DATA6</Data></Cell>
<Cell><Data ss:Type="String">DATA6</Data></Cell>
<Cell><Data ss:Type="String">DATA7</Data></Cell>
</Row>
[download]

Prints:

<Data ss:Type="String">DATA1</Data>
<Data ss:Type="String">DATA2</Data>
<Data ss:Type="String">DATA3</Data>
<Data ss:Type="String">DATA4</Data>
<Data ss:Type="String">DATA5</Data>
<Data ss:Type="String">DATA6</Data>
<Data ss:Type="String">DATA6</Data>
<Data ss:Type="String">DATA7</Data>
[download]