$item->{Categories} = 'Blue Category, This is another, This is one category, and green category too';
   # Note the syntax here requires curlies, to SET the property
   # You can omit the curlies when GETTING the properties, which I think is safer. 
   #Eg: $Contacts->Items  , instead of $Contacts->{Items}

   $item->Save; # This is the step you were looking for !