Try to create logical database requests, wrap those in perl subroutines, and place them in a separate module. This will greatly improve the readability of the main script, and will also improve maintainability.

Avoid "select * ..." SQL statements. Be explicit in what you are querying, even if that makes the code more verbose - it'll again improve maintainability.

One thing that is repeating a lot in the code is prepare/execute/fetch one row. Maybe you could use a utility subroutine that does that, something like:

sub fetch_one_row {
    my $sql = shift;
    my $sth = $dbh->prepare($sql);
    $sth->execute;
    $sth->fetchrow_hashref;
}
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Michael