There are two solutions to your problem.

The first will work, has already been suggested, but isn't going to give you much mileage past this particular case. That is, to shift the scalar out of @_ before assigning the remaining contents of @_ to your hash.

But the proper way to pass more than one complex variable (such as an array or a hash) into a subroutine is to pass a reference to the actual entity, rather than the entity itself. That is done like this:

my %new_stuff = ( "RATING" => 4.99 );
db_modify_account( "jdonald", \%new_stuff );

sub db_modify_account {
    my( $uid, $ref_new_data ) = @_;
    while ( ( $key, $value ) = each %{ $ref_new_data } ) {
        print "The $key is: $value.<br />";
    }
    print "Oh, and the user is $uid.<br />";
}
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The thing to keep in mind is that all arguments passed to a sub get flattened out into a single list. Also remember that a hash, in list context, becomes a flat list of key, value, key, value, etc.

So your current implementation passes the following list to the sub: "jdonald", "RATING", 4.99. And when the hash is reconstructed, under your current implementation, it is mapped out like this: jdonald => RATING, 4.99 => undef

Passing the references allows you to avoid the trickery it would take to figure out how to rebuild the complex data into a hash, array, or whatever. Imagine trying to pass two hashes, or two arrays as flat lists, and figuring out where one ends and the next begins. Passing references is a whole lot easier.

This is discussed in a bit more detail in perlsub, perlref, and perlreftut.