You would have to put a backslash in front of it normally, but because you've prototyped your sub Perl takes the reference for you behind the scenes and you receive that ref in $_[0] rather than the normal behavior of the hash being flattened into a list and you getting the key/value pairs in @_.

Of course one might feel inclined to yet again comment on the undesirability of prototypes, but I'm getting weary of doing so . . .