This is correct/expected behavior, and isn't Perchance-specific, but I'll admit it's confusing. It's basically "JavaScript labelled statements strike again". This is valid JavaScript:

{
  let a = 10;
  x:2
}

The curly braces define a block, so that the a variable is only defined within that block - this is very handy, and I use it quite a lot. It's equivalent to:

if(true) {
  let a = 10;
  x:2
}

The x:2 on the other hand is unusual and wouldn't be written in "normal" code. It's a labelled statement, but it's a useless one, since there's no way to actually "use" it.

So TL;DR: {x:2} is being treated as a block with a labelled statement rather than a JavaScript object. The workaround is to wrap it in parentheses like [ ({ x:1 }) ] instead of [ { x:1 } ].

I've added some comments to your example generator:

b = [ {} ] // a block with nothing in it
d = [ { x:1 } ] // a block with a labelled statement, x:1
e = [ x = {} ] // assignment, and so right hand side is constrained to be an expression - i.e. cannot be a block
f = [ { toString: function f() { return "hey"; } } ] // same as d
g = [ x2 = { toString: function f() { return "hey"; } } ] // same as e

You can test all of the above using eval like: eval("{x:1}") to see it's not Perchance-specific. To add to the confusion, DevTools does some "magic" on top of eval in order to basically prioritize interpretation as an object, I think.

I like DevTools' approach better, and have actually considered "fixing" this, since it would be weird to use JS blocks with labelled statements inside square blocks, but it's probably too late due to backwards-compat issues, and would need to wait for V2 of the engine.

Even though it's not technically a bug, I'll link this comment from perchance.org/known-bugs as something to improve on in V2 of the engine.