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 m   Treat string as multiple lines.  That is, change "^" and "$" from
     matching the start or end of the string to matching the start or
     end of any line anywhere within the string.

$ perl -Mre=debug -wle'"ABC\nDEF" =~ m/C(?:$)^D/m'
Freeing REx: `","'
Compiling REx `C(?:$)^D'
...
Did not find anchored substr "CD"...
Match rejected by optimizer
Freeing REx: `"C(?:$)^D"'

$ perl -Mre=debug -wle'"ABC\nDEF" =~ m/C(?:$)$\?^D/m'
Freeing REx: `","'
Omitting $` $& $' support.
...
D> <EF>         | 11:    END
Match successful!
Freeing REx: `"C(?:$)\n?^D"'