From the statement "check the return value ... against 'undef'," do you mean you are doing something like:

my ($row1,...) = $sth->selectrow_array;
if( $row1 == undef ) {
    $row1 = 0;
}
# or
$row1 = 0 if $row1 == undef;
[download]

If you are, you may want to try something like:

if( not defined $row1 ) {
    $row1 = 0;
}
# or
$row1 = 0 if not defined $row1;
[download]

    -Bryan