$ perl -Mre=debug -wle'"ABC\nDEF" =~ m/C(?:$)^D/m'
Freeing REx: `","'
Compiling REx `C(?:$)^D'
size 7 Got 60 bytes for offset annotations.
first at 1
rarest char D at 1
   1: EXACT <C>(3)
   3: MEOL(4)
   4: MBOL(5)
   5: EXACT <D>(7)
   7: END(0)
anchored "CD" at 0 (checking anchored) minlen 2
Offsets: [7]
        1[1] 0[0] 5[1] 7[1] 8[1] 0[0] 9[0]
Omitting $` $& $' support.

EXECUTING...

Guessing start of match, REx "C(?:$)^D" against "ABC
DEF"...
Did not find anchored substr "CD"...
Match rejected by optimizer
Freeing REx: `"C(?:$)^D"'