Honestly, it all starts going to shite after "hello world."

The flight software for the Apollo moonshots.

You may be interested by this https://en.wikipedia.org/wiki/Formal_verification.

Prominent examples of verified software systems include the CompCert verified C compiler and the seL4 high-assurance operating system kernel.

Automotive engine control computers.

They just work, for decades and millions of miles.

There was a moment in time where maybe it was qmail:

https://en.wikipedia.org/wiki/Qmail

Ten years after the launch of qmail 1.0, and at a time when more than a million of the Internet’s SMTP servers ran either qmail or netqmail, only four known bugs had been found in the qmail 1.0 releases, and no security issues.

More on how it was accomplished:

https://blog.acolyer.org/2018/01/17/some-thoughts-on-security-after-ten-years-of-qmail-1-0/

No; since every user defines the perfect program differently. Which should be the default behaviour(s)?

I wanted to say VLC because to me, it's the gold standard of fully working open-source software that just destroys the commercial competitors.

But it's not perfect only because society changes. New video formats forces VLC and open-source devs to adapt. Bigger video and new tech specs require VLC to update. If it wasn't for all those external needs, VLC would be perfect.

Did I also mentioned the many times rich companies wanted to buy VLC and they laughed?

TempleOS

Of course: https://github.com/kelseyhightower/nocode

Is there a perfect building?

Probably not, since they exist in an environment — which is constantly changing — and are used by people — whose needs are constantly changing.

The same is true of software. Yes, programs consist of math which has objective qualities. But in order to execute in the physical world, they have to make certain assumptions which can always be invalidated.

Consider fast inverse sqrt: maybe perfect, for the time, for specific uses, on specific hardware? Probably not perfect for today.

Notepad.exe, pre-windows 11. Now it's something else entirely but still uses the name :(

TeX. Best documented source, and last bug found was 12 years ago.

7zip?

TeX?

Development is considered to be complete, and the version numbering is just adding a digit of pi. Last change was 5 years ago.

Yeah you probably can't do to much more to pwd or yes or whatever (yeah I know about the silly optimisations). I think once you get much beyond that there are always more features you can add. Even for something like cd, people have made fancier versions with fuzzy matching and so on.

wget.

Ski Free

LaTeX

I don't think such thing as perfect software exist, only abandoned software. If the environment changes, then the software needs changes too.

I would have said Windows notepad but they screwed the pooch on that one and changed it.

A program that just prints "Hello World" to the screen and quits.

For software to be perfect, can not be improved no matter what, you'd have to define a very specific and narrow scope and evaluate against that.

Environments change, text and data encoding and content changes, forms and protocol of input and output changes, opportunities and wishes to integrate or extend change.

pwd seems simple enough. cd I would already say no, with opportunities to remember folders, support globbing, fuzzy matching, history, virtual filesystems. Many of those depend on the environment you're in. Typically, shells handle globbing. There's alternative cd tools that do fuzzy matching and history, and virtual filesystems are usually abstracted away. But things change. And I would certainly like an interactive and fuzzy cd.

Now, if you define it's scope, you can say: "All that other stuff is out of scope. It's perfect within it's defined target scope." But I don't know if that's what you're looking for? It certainly doesn't mean it can't be improved no matter what.

Windows event viewer... You open it, go to the toilet, to the shower, take a coffee, ... and only 2 more minutes later, it shows you the entries...

It's so perfect, they never had to improve it in decades.

/s

WinRAR free version

Ed. It's the standard text editor.

https://www.gnu.org/fun/jokes/ed-msg.html

Pretty subjective but if you're looking for do one thing and do it well I'd go with some of the GNU core utils like you mentioned, vlc & ffmpeg for AV media, and sl for being a silly way to handle ls typos

Logic gates?

Depends on your definition of "perfect" and "improved". Is it perfect because it does one fundamental thing really well? Is it improved by adding new features?

I think what you're meaning is, is there a program that is ubiquitous (or at least works anywhere), will basically remain used forever because it does a fundamental job that will always need to be done, and it does that job in the most straightforward way possible that can't be made any algorithmically simpler, faster, etc. Probably plenty, honestly. Bitwise operations, arithmetic, fetch/store, etc. Though ubiquity/working anywhere gets rarer the higher you go from hardware. Even your suggestion of cd, for example, has to interface with an OS's file system, of which there are several common types. What it's doing is simple in concept, but will always be dependent on other programs for the file system.