FWIW the "possessive modifier" is simply syntactic sugar for (?>...) so (?>x+) should produce exactly the same optree as x++.

$ perl -Mre=debug -e'/x++/'
Compiling REx "x++"
Final program:
   1: SUSPEND (8)
   3:   PLUS (6)
   4:     EXACT <x> (0)
   6:   SUCCEED (0)
   7: TAIL (8)
   8: END (0)
anchored "x" at 0 (checking anchored) minlen 1 
Freeing REx: "x++"
$ perl -Mre=debug -e'/(?>x+)/'
Compiling REx "(?>x+)"
Final program:
   1: SUSPEND (8)
   3:   PLUS (6)
   4:     EXACT <x> (0)
   6:   SUCCEED (0)
   7: TAIL (8)
   8: END (0)
anchored "x" at 0 (checking anchored) minlen 1 
Freeing REx: "(?>x+)"
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so there is nothing stopping you using it in 5.8.8