Question 1: grep `perldoc DBI` for "undef":

undef NULL values are represented by undefined values in Perl

You might check like:

$employee_id = (defined $row[0]) ? $row[0] : '';
[download]

Question 2: do a select for that row. Using fetchrow_array(), it will return an empty list if there wasn't a row, so

@foo = $sth->fetchrow_array();
if (@foo) {
...
} else {
...
}
[download]

I use MySQL, so maybe Oracle has fancier features, for which read the manual.