It's all changing the context you are in, you can reference an element as @foo[$i] or $foo[$i], it's just an array slice of one (Ie. array context) versus a single element (scalar context). Eg.

my @abcd = (1,2,3);
print "@abcd" . "\n";
$abcd[0] = 4;
@abcd[1] = 5;
print "@abcd" . "\n";
$abcd[0] = (4, 5, 6);
@abcd[1] = (4, 5, 6);
print "@abcd" . "\n";
print @abcd . "\n";
__DATA__
1 2 3
4 5 3
6 4 3
3
[download]

As you can see the simple assignments are the same, it's only when you get to a point where the context matters that it changes.