use JSON::XS;
my @kwlist = qw(foo bar baz qux);
my @items = getfromdb("select id,keywordhash from table"); # I have a sub that does this
foreach my $i (0..$#items) {
    my ($id,$temp) = @{$items[$i]};
    my $kws = decode_json($temp);
    my $count = 0;
    foreach my $k (@kwlist) {
        if (defined($kws->{$k})) {
            $count++;
        }
    }
}