Just a wild guess (I don't have a Windows system / cpio here to try).

Maybe cpio doesn't like the \r that the file handle's :crlf layer (by default active on Windows) is adding when you say print $WRITEME "$f\n";.

At least on Unix I get similar stat errors when I deliberately add \r to the filenames piped to cpio — not sure, though, how a Windows version of cpio would handle carriage returns...

In other words, try binmode $WRITEME.