in reply to Calling perl systems from other systems, e.g. R

Depending on the complexity of the interaction between R and Perl that you desire, R packages can include Perl scripts (subject to availability of Perl on the target machine, of course, e.g. gtools::read.xml, gtools::installXLSXsupport), or R itself can work as a server accepting commands from clients (e.g. Rserve).

If you take the HTTP server way, please be careful about accepting arbitrary code to evaluate over the local socket. Far too many modern applications start HTTP servers on localhost, only for someone else to discover that there's an unprotected endpoint that can be trivially accessed by an evil website by sending a few thousand requests of the form http://localhost:${port}/eval?system("pwn_the_machine"). Ideally, I would suggest UNIX domain sockets and Windows named pipes (which are easily restricted to the current user and cannot be accessed by rogue JavaScript in a browser), but getting R to speak those (especially the latter) can be hard.