First, if you are using Win32::GUI, you shouldn't need to import SendMessage() yourself.

Second, you cannot just make **it up as you go along :) The description of the TVM_GETNEXTITEM message from the page I linked is:

lResult = SendMessage(         // returns LRESULT in lResult     
   (HWND) hWndControl,         // handle to destination control     
   (UINT) TVM_GETNEXTITEM,         // message ID     
   (WPARAM) wParam,         // = (WPARAM) (UINT) flag;    
   (LPARAM) lParam         // = (LPARAM) (HTREEITEM) hitem;    
);
[download]

The wparam argument should be one of the following flags, not the length of a buffer

TVGN_CARET
TVGN_CHILD
TVGN_DROPHILITE
TVGN_FIRSTVISIBLE
TVGN_LASTVISIBLE
TVGN_NEXT
TVGN_NEXTVISIBLE
TVGN_PARENT
TVGN_PREVIOUS
TVGN_PREVIOUSVISIBLE
TVGN_ROOT
[download]

And the lparam argument should be either null or the handle returned from a previous call, not a pointer to a buffer.

The basic mechanism is,

1. You pass TVGN_ROOT/null, to get a handle to the root item in the display.
2. Then you pass TVGN_CHILD/roothandle (from above) to get the handle to the first child.
3. 1. Then you use TVGN_NEXT/firstChildHandle (from above) to iterate the first level children.
   2. And TGVN_CHILD/firstChildHandle to process the grandchildren...

Obviously, you will need to loop/recurse at each level to optain all the siblings & children at each level. It's upto you to decide whether you depth first or breadth first.

The return value from the SendMessage is the item handle to each node in the tree. As well as using this as an argument to get the next sibling or child in the tree, you also use it to send TVM_GETITEM messages to each of those items to retrive the TVMITEM(EX) structure that contains the data & state that is the node.