Forth seems to be one of those write once languages like perl. Easy to start writing and building, but come back to the code in a year or so, no clue what it does.

But really fast and efficient.

Perl's reputation for this is entirely unfair, in my opinion.

I just opened up a random reasonably-large Perl program that I last touched over three years ago, browsed through the source, and I have absolutely no trouble telling what it does.

I don't think Perl code is any more difficult to revisit than code in most other programming languages are. I think the difficulty mostly rests with how proficient the developer is in the language, and how long it has been since they wrote any code in that language. If you are really rusty with the programming language, or you never really had a high level of proficiency in it in the first place, you are going to have a hard time revisiting the code no matter what language it is.

Of course, it will be harder if the code is not your own. Also, it depends a great deal on how well the code was written. There's plenty of garbage code out there, in any language you might use. And if it was garbage to begin with, it won't get any better with age.

Anecdotally I find the opposite to be the case. I think Forth is hard to write but easy to read.