Getting Blog Posts from Multiple Categories

Hi,

I'm trying to get a list of blog posts from a list of multiple categories.

My template for returning a list of categories for the current post looks like this:

"gallery/<perch:category id="catSlug" />",

This returns a string like this:

"gallery/forms","gallery/boots",

I then pass this into a perch_blog_custom function to return other results categorised in the same way, but not matchin the current post's slug:

<?php 
    $cat = perch_blog_post_categories(perch_get('s'), array(
            'template' => 'cat-name.html',
        ), true);

                if($cat<>''){

                    echo "<h2>Related Work</h2>";

                    $cat = substr($cat, 0, -1); //remove trailing comma

                    perch_blog_custom(array( 
                        'template'          => 'post_in_list-related.html',
                        'category'          => array($cat),
                        'category-match'    => 'any',
                        'filter'            => 'postSlug',
                        'match'             => 'neq',
                        'value'             => perch_get('s'),
                        'sort-order'        => 'DESC'
                    ));

                }

                ?>

This returns no results (should return 2).

If I copy the returned string ("gallery/forms","gallery/boots") directly into the category array in perch_blog_custom the function works.

Is there any reason why entering a list of categories into the array in this way (as a variable) shouldn't work?

Thanks!