my $res = $sth->execute;
if (! $res) {
    die "Couldn't execute >>$sql<< : " . $sth->errstr;
};

my $rows = 0;
while (my $pointer = $sth->fetchrow_hashref) {
    ... do magic ...
    $rows++;
};
if (! $rows) {
    print "No rows were affected.";
};