The sigil to use is the type being returned.

$foo             # Scalar
$foo[$i]         # Array element, a scalar
$foo{$k}         # Hash element, a scalar

@foo             # Array contents, a list of scalar values
@foo[$i, $j]     # Array slice, a list of scalar values
@foo{$k1, $k2}   # Hash slice, a list of scalar values

%foo             # Hash contents, a list of key-value pairs
[download]

$foo[EXPR] and @foo[EXPR] differ in the context in which EXPR is evaluated, and they differ in the number of scalars they can return.