$dbh = DBI->connect('..', '..', '..');
$sth = $dbh->prepare(q{select ..});
if ($sth && !$sth->errstr()) {
  while (my $row = $sth->fetchrow_arrayref()) {
    ..
  }
}

##</code><code>##

  
no strict 'refs';
*{ Test::MockDBI::errstr } = sub { return };

##</code><code>##

package db;

# db.pm

use strict;
use warnings;

use Data::Dumper;
use DBI;
our $dbh = undef;

sub connect {
  $dbh = DBI->connect('DBI:mysql:test', '', '') 
		or die "connect failed";
}

sub disconnect {
  $dbh->disconnect();
}

sub select_i_from_t {
  my $sth = $dbh->prepare(q{select i from t}) or die "prepare failed";
  $sth->execute() or die "execute failed";
  my $arref = [];
	if ($sth && !$sth->errstr()) {  # <-- errstr is always be false in
                                  # <-- Mocked mode 
		while (my $row = $sth->fetchrow_arrayref()) {
			push @$arref, $row->[0];
		}
	} else {
		die "no sth" if (!$sth);
		die "errstr:" . $sth->errstr() if ($sth->errstr());
	}
  return $arref;
}

1;

__END__

# db.t

use strict;
use warnings;

# Signal the we want to use Test::MockDBI
BEGIN { push @ARGV, "--dbitest=42"; }

use Data::Dumper;
use Test::More;
use Test::MockDBI;

BEGIN { plan tests => 4 }

use_ok(q{db});
my $mock_dbi = Test::MockDBI::get_instance();

# Replace the code for sub errstr in Test::MockDBI.
# Code is from Test::MockObject (CPAN)
{
	## no critic (ProhibitNoStrict)
	no warnings 'redefine';
	no strict 'refs';
	*{ Test::MockDBI::errstr } = sub { return };
}

my $vals = [ [123], [124], [125] ];
$mock_dbi->set_retval_scalar(
  42,
  "select i from t",
  sub { shift @$vals }
);

ok(defined(db::connect()), q{Expect connect() to return true});

is_deeply(
  db::select_i_from_t(),
  [ 123, 124, 125 ],
  q{Expect select_i_from_t() to return arrayref [ 123, 124, 125 ]}
);

ok(defined(db::disconnect()), q{Expect disconnect() to return true});


__END__

##</code><code>##

$ prove db.t
db....Name "Test::MockDBI::errstr" used only once: possible typo at db.t line 24.
db....ok 2/4errstr:DBI error at 'select i from t' at db.pm line 32.
# Looks like you planned 4 tests but only ran 2.
# Looks like your test died just after 2.
db....dubious
        Test returned status 255 (wstat 65280, 0xff00)
DIED. FAILED tests 3-4
        Failed 2/4 tests, 50.00% okay
Failed Test Stat Wstat Total Fail  Failed  List of Failed
-------------------------------------------------------------------------------
db.t         255 65280     4    4 100.00%  3-4
Failed 1/1 test scripts, 0.00% okay. 2/4 subtests failed, 50.00% okay.