$ perl -Mre=debug -E'qr/(a) \1 1/x'
Compiling REx "(a) \1 1"
Final program:
   1: OPEN1 (3)
   3:   EXACT <a> (5)
   5: CLOSE1 (7)
   7: REF1 (9)
   9: EXACT <1> (11)
  11: END (0)
anchored "a" at 0 floating "1" at 1..2147483647 (checking floating) minlen 2 
Freeing REx: "(a) \1 1"

$ perl -Mre=debug -E'qr/(a)(?:\1)1/'
Compiling REx "(a)(?:\1)1"
Final program:
   1: OPEN1 (3)
   3:   EXACT <a> (5)
   5: CLOSE1 (7)
   7: REF1 (9)
   9: EXACT <1> (11)
  11: END (0)
anchored "a" at 0 floating "1" at 1..2147483647 (checking floating) minlen 2 
Freeing REx: "(a)(?:\1)1"