What programming language requires 'a lot' of boilerplate code to use multiple files? That sounds awful. I don't think the argument for things being easier to find goes up either, with a tool like grep.
$ wc -l perl/lib/Sys/Guestfs.xs
11930 perl/lib/Sys/Guestfs.xs
Worse still, this expands to C which can be large and takes a noticable time to compile: 30019 perl/lib/Sys/Guestfs.c