and requires only one compiled regexp per pattern, instead of many as the current scheme dictates (embedding a qr// in a larger pattern results in a complete recompile).
It was my understanding that this wasn't true - that no recompile took place when nesting with (??{}), and testing seems to bear this out:
$ 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"'
