The Ready action in the PHB specifies that you can specify what circumstances will trigger it, and that when the trigger occurs, you can take your reaction or ignore the trigger. Further, if the action doesn't occur, you don't do anything. The only written condition on the trigger is that it is a "perceivable circumstance."

It seems to me that, RAW, you could always use the trigger "If I perceive literally anything happening," and choose to ignore the trigger whenever you don't feel like using the readied action -- effectively triggering whenever you want. Is this a valid condition for the Ready action? If so, it seems as if the further conditions on the Ready action are unnecessary, and as if you could just always use that trigger rather than specifying a specific condition, but that seems unlikely.

The perceivable circumstance must be particular.

You've missed some context from the rules for the Ready action (emphasis mine):

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."

The trigger must be both perceivable and particular. Though "literally anything happening" is technically something you can perceive, it's not particular, given that it would trigger immediately on the same turn you take the Ready action (the world doesn't stop being full of perceptible happenings on your turn). It also doesn't fit the examples given at face value, each of which depends on obvious occurrences.

Although there's no explicit metric of specificity, the trigger you propose definitely isn't valid. The examples provide a rough estimate for triggers that most DMs should consider reasonable. Aim for the thought process of a real person making a cause-and-effect decision about the world around them.

There's sometimes a cost to ignoring the trigger.

You sometimes have to expend a resource just to take the Ready action, even if you end up ignoring the trigger, such as when you Ready to cast using a spell using a spell slot (continued from the passage above, emphasis mine):

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger... When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.

If don't use your reaction to release the spell before the start of your next turn, the spell slot is wasted. There may be other features that have a similar up-front cost when using the Ready action.

It seems to me that, RAW, you could always use the trigger "If I perceive literally anything happening," and choose to ignore the trigger whenever you don't feel like using the readied action -- effectively triggering whenever you want. Is this a valid condition for the Ready action?

This might be a valid trigger, depending on your reading of the semantics of "particular circumstance". But it barely matters, because it's a spectacularly useless one. The rule says:

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

Assuming your character has senses and is experiencing the passage of time, this will happen immediately, at which point you will have to decide to take your declared action or not. There is no "reset, and wait until next instance of the trigger".

To pick a more normal example: if you say "When someone comes through the door, I shoot 'em", and then your buddy comes through the door followed by an orc a second later, you can shoot your buddy or not — but you can't ignore that trigger and then still have the action readied to shoot the orc.

Your plan is the same, except will go wrong immediately every time.

Or, in other words, there is not just sometimes a cost to readying an action, like the use of a spell slot or similar. There is always a cost — you use your action to ready for a circumstance, and if that situation doesn't happen or happens in a way you don't want to act on after all, you get nothing in exchange.

With your suggested trigger, immediately after your turn is over, you will perceive something, and therefore need to decide right then if you actually want to use your reaction to do the thing you planned. If you don't, the round moves on and you're done. This is equivalent to saying "I might shoot at the target. Yep, I do. That's my turn." or "I might shoot the target. Nope, wait, I decided not to. Skip my turn."