Because the parens just create a flat list. What you wrote is the same as:

my %hash = (
    'hi' => 'hello',
    'all' => 'bye',
    'later' => 'gone',
);
print $hash{'all'};
[download]

If you want nested hash references you need to use {}. Like this:

my %hash = (
    'hi' => {'hello' => 'all'},
    'bye' => {'later' => 'gone'},
);
[download]

For more on nested data structures read references quick reference.