You don't need to explicitly "use DBD::mysql" - DBI will handle that for you.

To help with debugging, wrap all your "prepares" and "executes" with eval, like this:

  eval {
    $sth = $self->dbh->prepare( $sql );
  };
  if ($@) {
    die "Database error (prepare): ", $self->dbh->errstr;
  }

  eval {
    $sth->execute();
  };
  if ($@) {
    die "Database error (execute): ", $self->dbh->errstr;
  }
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