OK, in 1935 Perl entered a joint venture with Gräf und Stift to build buses. This does not look like a pre-1935 G & S bus at all. Either Perl was way ahead of their time or the attribution to Perl (not Perl Auhof, that was post-war) is incorrect. The grille does look like 1938-39 G & S _cars._, so there might have been some parts sharing if this is indeed Perl.
Now. On a well-known transport site, I saw this photo labeled "1939 ÖAF Bus 3ton." I found it on another site labeled "Austro-Fiat HRM 6" (which you might remember was one of my guesses.) We know that G & S was taken over by ÖAF in 1971 but that's not pertinent to this puzzle. I am dubious about this late pre-war bus being either Perl or G & S. I found this Austro-Fiat brochure from 1937 on an auction site and this looks quite clearly like the puzzle bus. (except for the grille vent doors, which are shown on A-F trucks in the same brochure.). Now, if you take the mystery ÖAF or Austro-Fiat picture and flip it, it looks very much like the puzzle bus. Grille, vents, wheels, spare wheel, cloth roof, etc. So if this is indeed a Perl, it has a LOT of A-F parts. There may be some MAN parts too, as MAN had a large share of ÖAF after the Anschluss, and there was almost certainly a diesel engine licensed from MAN.
Edit: the picture is from the 2 & 2-1/2 ton "SRM" brochure. There is a 3 & 3-1/2 ton "HRM" brochure which shows a larger bus and vent doors on the hood but the SRM looks more like the puzzle bus. I suspect the 21-passenger body could be put on either chassis.
