Here's an XS file for a module and package My::Mod.
It contains a function named foo() that recursively calls itself until the supplied integer argument has been incremented to 20 (whereupon it returns the value '20').
If the supplied integer argument is >= 20, it returns that value immediately ... and if the supplied integer argument is a *lot* less than 20, then it might take a while to complete.

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"


int foo(int x) {
    if(x >= 20) return x;
    x++;
    x = foo(x);
}

MODULE = My::Mod    PACKAGE = My::Mod    

PROTOTYPES: DISABLE


int
foo (x)
    int    x
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Cheers,
Rob