You are making things really really difficult which HTML::Template can make simple.

You don't need any of that <CSTM> nonsense, or the filter callback.

Here's a template which does all you want:

<table>
    <tmpl_loop name="rows">
        <tr 
        <tmpl_if name="__odd__">
                class="odd" 
            <tmpl_else>
                class="even" 
            </tmpl_if>
            > 
            <td>
            <tmpl_var name="data">
            </td>
        </tr>
    </tmpl_loop>
</table>
[download]

You put the odd-even logic inside the opening TR tag and HTML::Template does the rest. The source code's going to look a bit weird but the browser doesn't care.