# get dates w/dups, and the dup count
SELECT `date`, count(*) as num_dups FROM foo GROUP BY `date` HAVING count(*) > 1

# get everything for one date
SELECT * FROM foo WHERE `date` = ?

# gather the unique combinations for each date.
my %dates;
my $rows = $dbh->selectall_arrayref("select * from foo", {Slice=>{}}, ); # (might not want to suck it all in at once, though)
foreach my $row (@$rows){
  push @{ $dates{ $row->{date} } }, $row;
}
print Dumper \%dates;