You will want to use a placeholder on the update statement for efficiency (especially with Oracle):

my $dbh = DBI->connect(...,{RaiseError=>1});

$asql = <<EOT;
SELECT  batch_line, row_id
FROM test_batch_lines
WHERE exe_c = 'N'
EOT
my $sel_h = $dbh->prepare($asql);

$bsql = <<EOT;
UPDATE test_batch_lines
SET exe_c = 'Y'
WHERE exe_c = 'N'
AND row_id = ?
EOT
my $upd_h = $dbh->prepare($bsql);

$sel_h->execute;
$sel_h->bind_columns(\my ($batch_line, $row_id));

while ($sth->fetch) {
 # ...
 $upd_h->execute($row_id);
}
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