$ perl -we'use re "debug"; $x=qr/(.)\1|(.)(??{$x})\2/; "abcddcba" =~ /^$x\z/'
Compiling REx `(.)\1|(.)(??{$x})\2'
size 20 Got 164 bytes for offset annotations.

   1: BRANCH(9)
   2:   OPEN1(4)
   4:     REG_ANY(5)
   5:   CLOSE1(7)
   7:   REF1(20)
   9: BRANCH(20)
  10:   OPEN2(12)
  12:     REG_ANY(13)
  13:   CLOSE2(15)
  15:   LOGICAL[2](16)
  16:   EVAL(18)
  18:   REF2(20)
  20: END(0)
minlen 1 with eval 
Offsets: [20]
	0[0] 1[1] 0[0] 2[1] 3[1] 0[0] 4[2] 0[0] 6[1] 7[1] 0[0] 8[1] 9[1] 0[0] 17[0] 17[0] 0[0] 18[2] 0[0] 20[0] 
Compiling REx `^(?-xism:(.)\1|(.)(??{$x})\2)\z'
size 23 Got 188 bytes for offset annotations.
first at 2
   1: BOL(2)
   2: BRANCH(10)
   3:   OPEN1(5)
   5:     REG_ANY(6)
   6:   CLOSE1(8)
   8:   REF1(22)
  10: BRANCH(21)
  11:   OPEN2(13)
  13:     REG_ANY(14)
  14:   CLOSE2(16)
  16:   LOGICAL[2](17)
  17:   EVAL(19)
  19:   REF2(22)
  21: TAIL(22)
  22: EOS(23)
  23: END(0)
floating `'$ at 1..2147483647 (checking floating) anchored(BOL) minlen 1 with eval 
Offsets: [23]
	1[1] 9[1] 10[1] 0[0] 11[1] 12[1] 0[0] 13[2] 0[0] 15[1] 16[1] 0[0] 17[1] 18[1] 0[0] 26[0] 26[0] 0[0] 27[2] 0[0] 28[0] 30[2] 32[0] 
Guessing start of match, REx `^(?-xism:(.)\1|(.)(??{$x})\2)\z' against `abcddcba'...
Found floating substr `'$ at offset 8...
Guessed: match at offset 0
Matching REx `^(?-xism:(.)\1|(.)(??{$x})\2)\z' against `abcddcba'
  Setting an EVAL scope, savestack=15
   0 <> <abcddcba>        |  1:  BOL
   0 <> <abcddcba>        |  2:  BRANCH
  Setting an EVAL scope, savestack=21
   0 <> <abcddcba>        |  3:    OPEN1
   0 <> <abcddcba>        |  5:    REG_ANY
   1 <a> <bcddcba>        |  6:    CLOSE1
   1 <a> <bcddcba>        |  8:    REF1
                              failed...
   0 <> <abcddcba>        | 11:    OPEN2
   0 <> <abcddcba>        | 13:    REG_ANY
   1 <a> <bcddcba>        | 14:    CLOSE2
   1 <a> <bcddcba>        | 16:    LOGICAL[2]
   1 <a> <bcddcba>        | 17:    EVAL
  re_eval 0x1003bb30
Entering embedded `(.)\1|(.)(??{$x})\2'
  Setting an EVAL scope, savestack=35
   1 <a> <bcddcba>        |  1:      BRANCH
  Setting an EVAL scope, savestack=41
   1 <a> <bcddcba>        |  2:        OPEN1
   1 <a> <bcddcba>        |  4:        REG_ANY
   2 <ab> <cddcba>        |  5:        CLOSE1
   2 <ab> <cddcba>        |  7:        REF1
                                  failed...
   1 <a> <bcddcba>        | 10:        OPEN2
   1 <a> <bcddcba>        | 12:        REG_ANY
   2 <ab> <cddcba>        | 13:        CLOSE2
   2 <ab> <cddcba>        | 15:        LOGICAL[2]
   2 <ab> <cddcba>        | 16:        EVAL
  re_eval 0x10019550
Entering embedded `(.)\1|(.)(??{$x})\2'
  Setting an EVAL scope, savestack=55
   2 <ab> <cddcba>        |  1:          BRANCH
  Setting an EVAL scope, savestack=61
   2 <ab> <cddcba>        |  2:            OPEN1
   2 <ab> <cddcba>        |  4:            REG_ANY
   3 <abc> <ddcba>        |  5:            CLOSE1
   3 <abc> <ddcba>        |  7:            REF1
                                      failed...
   2 <ab> <cddcba>        | 10:            OPEN2
   2 <ab> <cddcba>        | 12:            REG_ANY
   3 <abc> <ddcba>        | 13:            CLOSE2
   3 <abc> <ddcba>        | 15:            LOGICAL[2]
   3 <abc> <ddcba>        | 16:            EVAL
  re_eval 0x10019550
Entering embedded `(.)\1|(.)(??{$x})\2'
  Setting an EVAL scope, savestack=75
   3 <abc> <ddcba>        |  1:              BRANCH
  Setting an EVAL scope, savestack=81
   3 <abc> <ddcba>        |  2:                OPEN1
   3 <abc> <ddcba>        |  4:                REG_ANY
   4 <abcd> <dcba>        |  5:                CLOSE1
   4 <abcd> <dcba>        |  7:                REF1
   5 <abcdd> <cba>        | 20:                END
  Setting an EVAL scope, savestack=95
     restoring \1 to 2(2)..-1
     restoring \2 to 2(2)..3
   5 <abcdd> <cba>        | 18:                  REF2
   6 <abcddc> <ba>        | 20:                  END
  Setting an EVAL scope, savestack=109
     restoring \1 to 1(1)..-1
     restoring \2 to 1(1)..2
   6 <abcddc> <ba>        | 18:                    REF2
   7 <abcddcb> <a>        | 20:                    END
  Setting an EVAL scope, savestack=123
     restoring \1 to 0(0)..-1
     restoring \2 to 0(0)..1
   7 <abcddcb> <a>        | 19:                      REF2
   8 <abcddcba> <>        | 22:                      EOS
   8 <abcddcba> <>        | 23:                      END
Match successful!
Freeing REx: `"^(?-xism:(.)\\1|(.)(??{$x})\\2)\\z"'
Freeing REx: `"(.)\\1|(.)(??{$x})\\2"'