+=item * String interpolation of C<$#array> differs
+
+String interpolation of the C<$#array> construct differs when braces
+are to used around the name.
+
+ @a = (1..3);
+ print "${#a}";
+
+ # perl4 prints: 2
+ # perl5 fails with syntax error
+
+ @ = (1..3);
+ print "$#{a}";
+
+ # perl4 prints: {a}
+ # perl5 prints: 2
+
+=item * Perl guesses on C<map>, C<grep> followed by C<{> if it starts BLOCK or hash ref
+
+When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{>
+starts a BLOCK or a hash reference. If it guesses wrong, it will report
+a syntax error near the C<}> and the missing (or unexpected) comma.
+
+Use unary C<+> before C<{> on a hash reference, and unary C<+> applied
+to the first thing in a BLOCK (after C<{>), for perl to guess right all
+the time. (See L<perlfunc/map>.)
+