foo (%hash);

sub foo
{
    my %passed_hash = @_;
    # do stuff
}

##</code><code>##

sub foo (\%$);

foo (%hash, $arg);

sub foo (\%$)
{
    my $hash_ref = shift;
    my $passed_arg = shift;

    my $passed_hash = %$hash_ref;

    # do stuff
}