Although it gets a bit hairy, one can explain that behavior by saying that my $a = (10,20,30); is merely an application of the comma operator in scalar context, whereas the rhs of my $a = (10,20,30)[0..2] is really a list, and so the behavior of taking the last value has to be explicitly defined. The distinction is important because I believe that  my $a = (1,2,foo) would call foo in scalar context, but my $a = (1,2,foo)[0..2] would call it in list context.

The preceding paragraph will be moot someday because according to Apocalypse 3, there will be no scalar comma operator in Perl 6 - it will always be a list, and a list in scalar context will return an array reference