# Two scripts in parallel:
my $highest_id1 = $dbh->do("select max(id) from my_table"); # first script
my $highest_id2 = $dbh->do("select max(id) from my_table"); # second script

$highest_id1++; # first script
$highest_id2++; # second script

if ($highest_id1 == $highest_id2) {
    die
};

##</code><code>##

create table my_table (
    id integer primary key not null,
    ....
);

-- or
create unique index idx_id on my_table (id);

-- or, in MySQL I think:
create table my_table (
    id integer not null auto_increment,
    ....
);