hash to sub

[Anonymous Monk]

Dear Monks,
as a newbie in Perl I use snippets of the others' code and try to understand them.
Now I have a piece of code which works but I do not understand why.
I pass arguments to a subroutine as follows (I need a populated hash in the subroutine):

sub marine($string, \%hash)

Then I take the parameter in the subroutine:

sub marine
{
my $line = shift;
my(%hash) = %{(shift)};
...
}

The line that I do not understand is my(%hash) = %{(shift)};. Why there are parenthesis there? Could you please explain this?
Thank you!