The __DATA__ (a.k.a. __END__) block doesn't do interpolation. (Update: Nor \u \U \l \L \Q \E \t \n \r \etc similarly.)

Also remember that by default, you're reading the lines using \n (newline) as a line delimiter, so if you should succeed in embedding \n into a line that you write to a file, you'll end up with two lines!