Anyway, draw a box on the canvas, make all angles in the toolbar parallel and set them to something like X: 185 Y: 270 Z: 350 (or X: 175 Y: 90 Z: 10 in older verions). Adjust your box with the diamond-shaped handles.
Switch to the selection tool, select your box and set a stroke in the 'Fill and Stroke' dialog, or select some of its faces with the node-editing tool and set the fill color.
Enable snapping, snapping to nodes and snapping to cusp nodes in the snapping toolbar. Now make a clone of your box with Alt + D (duplicates seem to constantly lose their perspective information) and drag it next to your original box until it snaps two corners together. Repeat until you have the amount of boxes in the shape you want. You may have to adjust the z-order with the 'raise' and 'lower' buttons in the toolbar.