Time for a crazy guess. This bus looks as though it could be on a modified ÖAF DT55 chassis. So, with Perl's standard designation scheme in mind... 'O 34'?
Not crazy at all. It doesn't use the ÖAF DT 55 chassis, but its engine. So it is not an O ... (and not a 34).
Locked for you.
Unfortunately, I lack both knowledge and resources on the subject of buses. I'm under the impression Perl modified donor chassis as opposed to building them from scratch. In this instance, the donor chassis was neither Steyr nor ÖAF. Was it Saurer? I may be mistaken, but it seems the letter component of Perl's bus designations usually (but not always) corresponded with the name of the chassis builder. If Steyr = 'St', then perhaps Saurer = Sa? As for the wheelbase, let's take that guess way down to 2600 mm. Which brings me to ... Perl-Auhof Sa 26?
If not, then please go ahead and unlock the thread. While I could keep guessing, someone else may be able to actually find the answer.