The cute thing here is that the hash is initialized with a list of one item, a reference to an empty hash. So:

$ perl -e '%foo = {}; print %foo, "\n";'
HASH(0x814dc30)
[download]

and (note the trailing comma, indicating an undefined value):

$ perl -e '%foo = {}; print join(",", %foo), "\n";'
HASH(0x814dc30),
[download]