I definitely don't prefer it this way. This was a project I started many, many years ago actually. It was one of the very first projects I ever thought of when I started writing Perl, and it took many years to put together. It was more a "can I do this" thing :)
As a matter of fact, another project I went on for the sake of learning how is monkey patching on the fly with my Mock::Sub and Wrap::Sub, so updating Devel::Trace::Subs is something I definitely could do to remove the actual writing to files. I'll create myself an issue so I don't forget to do this someday.