I assume you mean spoilers on perlmonks, or do you mean in general?
If you can use Javascript, it's easy to make a

<button onClick='alert("This'll spoil your day!");'>
[download]

Or using CSS, you might make a

<style>
.spoiler:hover
{
  visibility: visible;
}
.spoiler
{
  visibility: hidden;
}
</style>
...
<span class="spoiler">This'll spoil your week!</span>
[download]

</code>