It does the equivalent of grep /regex/, @_. => has most of the same properties of a comma.

For example

%hash = (this => 'that', now => 'then');
[download]

is equivalent to

%hash = ('this', 'that', 'now', 'then');
[download]

You can see how it differs from the comma in the quoting of the keys being optional in most cases. But for most normal uses, its just a funny looking comma.