structs come as either values or objects

The treatement of a compact struct as a value or as an
object depends on whether there are mutators.

S09-data.pod

@@ -13,8 +13,8 @@ Synopsis 9: Data Structures
 
     Created: 13 Sep 2004
 
-    Last Modified: 23 Sep 2010
-    Version: 49
+    Last Modified: 4 Jan 2011
+    Version: 50
 
 =head1 Overview
 
@@ -154,9 +154,10 @@ how to lay them out in memory, possibly with a pack/unpack format
 associated with the class, or with the strange elements of the class,
 or with the types under which the strange element is declared.
 
-Note that a compact struct is itself a value type, so except for
+Note that a compact struct that has no mutators is itself a value type, so except for
 performance considerations, it doesn't matter how many representations
-of it there are in memory as long as those are consistent.
+of it there are in memory as long as those are consistent.  On the other
+hand, structs with mutators must behave more like normal mutable objects.
 
 The packing serialization is performed by coercion to an appropriate
 buffer type.  The unpacking is performed by coercion of such a buffer