use Net::LDAP::Util qw(ldap_error_text
                       ldap_error_name) ;

my $msg ; # you will need it several times
$msg = $ldap->bind(%your_bind_params) ;
if ($msg->is_error) {
  my $code = $msg->code ;
  die join "\n",ldap_error_name($code),
                ldap_error_text($code) ;
}

##</code><code>##

while (my ($username,$userdata) = each %petra) {
  my $entry = Net::LDAP::Entry->new();
  my $dn = calculate_it_using($username) ;

  $entry->dn($dn) ;
  $entry->changetype("add");
  $entry->add(@$userdata) ;

  $msg = $entry->update($ldap) ;
  if ($msg->is_error) {
    # die() the same way as before. I'd suggest
    # to create a subroutine to do the job, e.g.
    # die_ldap_error($msg->code)
  }
}