Hi i must admit honestly that i dont understand your code completeley. But if i understand your problem right you want to:?

Create a list of directories (down the "directory-tree) starting from $sourcedir.
Store this list in an array e.g..
Loop (foreach array element e.g.):

{
Descent into every dir and copy *txt to $targetdir
}

(Special treatment required for "." and ".." directories (the current and the dir nearer to the "root-dir" these must be removed from the list)