Please know that I have no experience with Solaris, or threaded applications using XS to speak of. That said, it seems you are leaking file descriptors, as you are doing:

fd = open(path, O_RDWR);
[download]

and you're not closing the file while the "int fd" does go out of scope. This is XS, not Perl! The file will not be automatically closed when its "handle" is destroyed. This may or may not be related to what you're seeing, though.

Liz