my %id_to_name = (
    1 => 'keith',
    2 => 'rob',
    3 => 'eric',
    4 => 'rob',
    # etc.
);

my %id_to_mark = (
    1 => 90,
    2 => 52,
    3 => 86,
    4 => 71,
    # etc.
);

##</code><code>##

my %scores_to_names = (
    90 => [ 'keith' ],
    52 => [ 'rob', 'dave' ],
    # etc.
);