alter session set NLS_TIMESTAMP_FORMAT = 'RRRR-MM-DD HH24:MI:SS';
alter session set NLS_DATE_FORMAT = 'RRRR-MM-DD';
alter session set NLS_TIME_FORMAT = 'HH24:MI:SS';

##</code><code>##

my $dbh = ... connect ...;
$dbh->do("alter session set NLS_TIMESTAMP_FORMAT = 'RRRR-MM-DD HH24:MI:SS'");
... etc ...