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
-xdg
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