Apologies if you’re having trouble posting comments. Anything under 800 characters or so should work OK. Long posts are potentially problematic. If you have something longer you really need to say, send me an e-mail and I’ll post it.
Update 7-July: SixApart says there’s now a fix. I’ll have to wait till the CPanel update at my hosting company tonight to see whether it works. I’ve been getting the same old 500 errors today.
Update 8-July: Still getting those HTTP 1.1/500 errors. My hosting company installs nightly CPanel updates, so the fix wasn’t included with last night’s updates. When I checked the CPAN Web site yesterday, I noticed that there were two replacement files available for the file that causes the problem; one of the files was tagged as a Developer’s Version. Does that mean that it won’t be updated automatically? This sure is a PITA. I have to rebuild the entire site manually after every post longer than a few sentences, and comments are still flaky.
And no, I don’t blame Movable Type for this. The problem is caused by a Perl module distributed by someone else. But the problem does underscore the dependencies that are at the heart of any Web-based application.
Ed Bott 
Just as an update, Ed, we’re hearing that some hosts lag as far as 48 hours behind in their CPanel updates, which we unfortunately can’t control. If you’re still seeing the errors today, feel free to file a help ticket with our support system or get in touch.
The last post I made today went through just fine, so (fingers crossed) maybe it’s fixed.