In Clip Studio Paint, you can draw accurate perspective lines freehand using the [Perspective ruler].
How to download
① Select [Layer] menu -> [Ruler - Frame] -> [Create Perspective Ruler], then select the perspective view you want to create via the [Create Perspective Ruler] dialog box.
② If you use a drawing tool, such as the pen, you can draw a line freehand and it will snap into the straight line of the [Perspective ruler].
Also, you can create a [Perspective ruler] and a vanishing point that matches your underlying image using [Figure] tool -> [Create ruler] -> [Perspective ruler].
For details on how to use the [Perspective ruler] and the perspective view, please refer to this article.