in reply to Handling Null or Undef values in DBI (and things of those nature)...

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] : '';

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