The DBI documentation on cpan might help. It returns the number of rows affected on a successful query, and undef in the event of an error. If there are zero affected rows it returns "0E0", thus returning true for all successful queries. I snagged this off the documentation, which it says the default do method is logically similar to:

sub do {
      my($dbh, $statement, $attr, @bind_values) = @_;
      my $sth = $dbh->prepare($statement, $attr) or return undef;
      $sth->execute(@bind_values) or return undef;
      my $rows = $sth->rows;
      ($rows == 0) ? "0E0" : $rows; # always return true if no error
  }
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Hope this helps...