modify your code to this, note that this will only work with categories.

function modify_pop_recent_title( $title, $post ) {
return '<span class="pop-title title">'. $title . '<span class="post-date">' . date_i18n( get_option('date_format'), $date ) ,'</span>' . get_the_term_list( $post->ID, 'category', '', ', ', '' ) . '</span>' ;
}
add_filter('pop_recent_title', 'modify_pop_recent_title');