The original question was around spaces in identifier names; let me describe how I imagined that would go and you can pick holes in it:

- on disk, use (brackets around identifiers) so they parse

- but you don't want to have to keep typing those, so...

- have a smart IDE that manages identifier brackets for you

- display identifiers in special colours, fonts, etc

Maybe this is a slippery slope argument, but I think when you do that, you've taken a step towards visual programming, where the code is a special data structure and you can only feasibly interact with it via special tooling. I'm with the camp who think that gives you SmallTalk images or UML and it just doesn't work out.

An alternative would be not to put brackets around long identifiers, just have a smarter context-sensitive parser; but I think that's also a bad direction to go in. Putting lots of complexity into the parser (C++, Perl) is a bad idea and keeping the parser simple and regular is a good idea (Go, Python).

EDIT TO ADD:

The problem is that a tree of directories and files is clearly not a very good abstraction for structuring programs

Hmm, I disagree! It's not perfect, but I think it actually works pretty well. Files and folders map nicely onto modules and packages, and those are a reasonably effective way of organising the code for a big project.