my $dbh = ... get handle to database ...;
my $sth = $dbh->prepare("SELECT * FROM TABLE");
$sth->execute;
my @buffer;
my $seen;
while (my $row = $sth->selectrow_arrayref) {
    $seen++;
    if (@buffer < 100) {
        push @buffer, $row;
        next;
    }
    my $index = int rand $seen;
    if ($index < @buffer) {
        $buffer[$index] = $row;
    }
}