The docs don't come right out and say it, but @CLIPW doesn't append. Each @CLIPW starts at the beginning of the clipboard.
You can, of course, append with the likes of
But be careful. @CLIPW does not automatically put a newline in the clipboard either. So a subsequent ">> clip:" will likely be added at the end of the line written with @CLIPW.
Code:
v:\> echo %@clipw[1 First Line]
0
v:\> echo 2 Second Line >> clip:
v:\> type clip:
1 First Line2 Second Line