You don't mention whether you have AutoCommit set, but this is a common error when it's not. You might want to try something like this, assuming the data you want to update are arrayrefs in @data:

  $dbh->{RaiseError} = 1; # if not already
  my $sql = "UPDATE $table SET X = ? WHERE Y = ?";
  my $sth = $dbh->prepare( $sql );
  eval {
    foreach my $d ( @data ) {
      $sth->execute( $d->[0], $d->[1] );
    }
  };
  if ( $@ ) {
    print "Caught error: $@\nRolling back.\n";
    $dbh->rollback;
  }
  else {
    $dbh->commit;
  }
[download]

Chris
M-x auto-bs-mode