This doesn't work, because of the (documented in perlop) magic comparison with $. when the conditions are constants:

Fix:

perl -lwe'$true=1; print for grep $true../D/, qw(A B C D E F)'
[download]

Note that it will flip for "E" (like your last version). What you probably intended is:

perl -lwe'my @a=qw(A B C D E F); print for grep ($_==1) .. ($a[$_]=~/D
+/), 0..$#a'
[download]

Or without a flip-flop:

perl -lwe'for (qw( A B C D E F )) { print; last if /D/ }'
[download]