I still think it's /...(?:\1).../.

• In math and Perl, when one wants to group operators, one uses parens. Seems natural to use them here too.
• Enclosing in parens even has a precedent in Perl: /${prefix}_s/, /(?s:.)/
• It's self-contained instead of requiring unrelated code to change. This solves /...\1$pat.../.
• It's has no run-time effect (as shown below).

$ 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) mi
+nlen 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) mi
+nlen 2 
Freeing REx: "(a)(?:\1)1"
[download]

Update: Me tired. Grammar bad. Fixed.

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