I believe this format is typically written out from the Virtuoso line of tools. So, you could generate one from a simple IC design from that tool if you want an example. I can't recall ever seeing a published format spec for it, though you could probably find one with Google.
You've not mentioned if there's a strong reason to prefer this file format. The die text format, which is a simple CSV format file, is generally easier to write (and easier to modify as a user using a spreadsheet tool like Excel). Have you considered using that format, instead? It includes some capabiities that the D.I.E. file does not, too, like being able to embed the die's attachment type (wire bond vs. flip-chip), orientation, placed die rotation, die shrink and scribe information, etc.
Hope that helps.