Great suggestion! I forgot about regex debug mode. It looks like whatever the code in (??{ code }) evaluates to is compiled into a regex (if it isn't one already) and then that regex is applied in a separate scope from the rest of the regex -- so it has its own match variables and so on. I wouldn't have expected that given the documentation in perlre.

In the debug output, the telltales appear to be "Entering embedded..." and "restoring...", as seen below.

Entering embedded `(.)\1*'
  Setting an EVAL scope, savestack=31
   0 <> <xx556xx>         |  1:    OPEN1
   0 <> <xx556xx>         |  3:    REG_ANY
   1 <x> <x556xx>         |  4:    CLOSE1
   1 <x> <x556xx>         |  6:    CURLYX[1] {0,32767}
   1 <x> <x556xx>         | 10:      WHILEM[1/1]
                                0 out of 0..32767  cc=140fafc
  Setting an EVAL scope, savestack=37
   1 <x> <x556xx>         |  8:        REF1
   2 <xx> <556xx>         | 10:        WHILEM[1/1]
                                  1 out of 0..32767  cc=140fafc
  Setting an EVAL scope, savestack=43
   2 <xx> <556xx>         |  8:          REF1
                                    failed...
                                  failed, try continuation...
   2 <xx> <556xx>         | 11:          NOTHING
   2 <xx> <556xx>         | 12:          END
  Setting an EVAL scope, savestack=47
     restoring \1 to -1(0)..1(no)
     restoring \1..\1 to undef
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