# first, determine all the id's and which sets they appear in.
# presume that no id can appear more than once per set.
# also initialize the (one) record per id for the result.

my %by_id;
my %out_rec;

for my $set ( keys %in_hash )
{
    for ( @{ $in_hash{$set} } )
    {
        $by_id{$_->{'id'}}{$set} = $_;
        %{$out_rec{$_->{'id'}}} = %$_;
    }
}

my %out_hash;

for my $id ( keys %by_id )
{
    my @sets = sort keys %{ $by_id{$id} };
    $out_rec{$id}{'score'} = 0;
    $out_rec{$id}{'score'} += $by_id{$id}{$_}{'score'}
        for @sets;
    push @{ $out_hash{"@sets"} }, $out_rec{$id};
}

# and at this point %out_hash contains the desired data.
[download]