xbacklight doesn't work in any of my Puppys.
adjbacklight always seems to work.
https://github.com/maandree/adjbacklight
32 bit adjbacklight works in BionicPup64 (it doesn't need 32 bit compatibility layer)
"In fact it seems to work for all machines with backlight, which is not true for xbacklight"
In BionicPup64 and, I think, FossaPup, maybe, I use the included script brightness-set.