Perl regular expressions don't even have variables, so you couldn't possibly have shown one being used.
This is what I first thought you meant, but it didn't make any sense to me. If one distinguishes “regex variables” (variable at regex run-time) from “Perl variables” (constant at each regex run-time *), then, as you (basically) say, the only ones (I think) are the \1 back-references; but, when Hena said
I would assume that quantifier cannot be a '\x' variable
It's simpler than that: The quantifier cannot be variable.
The only reason I could think of to make this comment is if there were some other sort of variable, but you seem to be saying that there isn't. Did I misunderstand?
UPDATE: * False, as I finally realised in Re^6: Regex fun.