$ perl -Mre=debug -wle'"ABC\nDEF" =~ m/C(?:$)$\?^D/m'
Freeing REx: `","'
Omitting $` $& $' support.

EXECUTING...

Compiling REx `C(?:$)
?^D'
size 11 Got 92 bytes for offset annotations.
first at 1
rarest char D at 0
rarest char C at 0
   1: EXACT <C>(3)
   3: MEOL(4)
   4: CURLY {0,1}(8)
   6:   EXACT <\n>(0)
   8: MBOL(9)
   9: EXACT <D>(11)
  11: END(0)
anchored "C"$ at 0 floating "D" at 1..2 (checking floating) minlen 2
Offsets: [11]
        1[1] 0[0] 5[1] 8[1] 0[0] 7[1] 0[0] 9[1] 10[1] 0[0] 11[0]
Guessing start of match, REx "C(?:$)
?^D" against "ABC
DEF"...
Found floating substr "D" at offset 4...
Found anchored substr "C"$ at offset 2...
Starting position does not contradict /^/m...
Guessed: match at offset 2
Matching REx "C(?:$)
?^D" against "C
DEF"
  Setting an EVAL scope, savestack=6
   2 <AB> <C
DEF>         |  1:  EXACT <C>
   3 <ABC> <
DEF>         |  3:  MEOL
   3 <ABC> <
DEF>         |  4:  CURLY {0,1}
                           EXACT <\n> can match 1 times out of 1...
  Setting an EVAL scope, savestack=6
   4 <ABC
> <DEF>         |  8:    MBOL
   4 <ABC
> <DEF>         |  9:    EXACT <D>
   5 <ABC
D> <EF>         | 11:    END
Match successful!
Freeing REx: `"C(?:$)\n?^D"'