Perl's raw socket functions more or less mirror the underlying system calls. So when you're looking for documentation, you can Google something like "linux bind to interface", which would lead you to this Stack Overflow answer, which leads you to the SO_BINDTODEVICE option, which is documented in the socket(7) manpage.
However, from what I can tell the SO_BINDTODEVICE is not available on all OSes, and it's also not available in the Socket module. See this thread for how you could hack it.
On the other hand, why are you using the low-level functions, when there are higher-level modules such as IO::Socket and the Net:: family around? Also, I think it's much more common to bind your socket to a specific address instead of directly to an interface. Why do you need to bind to a specific interface?
