Edit in JSFiddle

<nav class="mega-menu">
    <ul>
        <?php 
          /*
          we grab all the data from our content region and assign it to a variable with a  "skip-template" parameter set to "true". It will prevent from rendering the menu for now.
          */
          $menu_custom = perch_content_custom('Mega Menu',array(
            'skip-template' => true
          ), true);

          // lets go through every menu item 
          foreach ($menu_custom as $key => $menuitem) {
 
            // for every item that has a url defined we output a usual link for our navigation */
            if(isset($menuitem['navitemhref'])){
              echo "<li><a class='mega-menu-link' href='" . $menuitem['navitemhref'] . "'>" . $menuitem['navitem'] . "</a></li>";
            } else {
                // otherwise let's have a static label that will act as a drop down trigger 
                echo "<li class='mega-menu-dropdown'><span class='mega-menu-link'>" . $menuitem['navitem'] . "</span><div class='mega-menu-col-wrapper'>";

                // now let's run a loop through all of Perch Blocks and identify whether there is a column or a column item that we are dealing with

                foreach ($menuitem['_blocks'] as $key => $dropdownitem) {

                  if(isset($dropdownitem['collabel'])) {
                    // we also need to look out where column starts and finishes
                    if($key !== 0) {
                      echo "</ul></div><div class='mega-menu-col'><p class='mega-menu-link'>" . $dropdownitem['collabel'] . "</p>
                      <ul>";
                    } else {
                      echo "<div class='mega-menu-col'><p class='mega-menu-link'>" . $dropdownitem['collabel'] . "</p>
                      <ul>";
                    }
                  } else {
                    echo "<li class='mega-menu-item'><a href='" . $dropdownitem['colitemhref'] . "'>" . $dropdownitem['colitem'] . "</a></li>";
                  }
                  
                }
                echo "</div></li>";
            }
        }
        ?>
    </ul>
</nav>