The package variables ($DBI::err and $DBI::errstr) are very shortlived because they are changed everytime an access is made to the DBI (well nearly everytime).

Thus if you have two statements then this

$sth1->execute;
$sth2->execute;
print "Statement 1 failed" if $DBI::err;
[download]

is wrong, as $DBI::err contains the error from $sth2.

The DBI book (and the DBI pod) therefore recommends that you use $sth1->err and $sth2->err instead if possible. (They are marignally faster to access as well).