# Call srand if you are on an old perl (< 5.004)

my $statement = 'SELECT ssn FROM ...';
my $sth = $dbh->prepare($statement);
$sth->execute();

my $target_lines = 1000;
my @result = ();
my $item_num = 1;
while (my ($ssn) = $sth->fetchrow_array()) {
    if (@result < $target_lines) {
        # Just add it if we have not hit the limit
        push @result, $ssn;
    }
    elsif (rand($item_num) < $target_lines) {
        # Otherwise pick something to replace if the
        # the odds are right
        $result[rand($target_lines)] = $ssn;
    }
} continue {
    $count++;
}