Here is the C code used for the above example, including build steps.

The header file (xswrap.h)

int mult (int x, int y);
void speak (const char* str);
unsigned char* arr ();
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The .c file (xswrap.c)

#include <stdio.h>
#include <stdlib.h>

int mult (int x, int y){
    return x * y;
}

void speak (const char* str){
    printf("%s\n", str);
}

unsigned char* arr (){
    unsigned char* list = malloc(sizeof(unsigned char) * 3);

    int i;

    for (i=0; i<3; i++){
        list[i] = i;
    }

    return list;
}
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The entry point file (main.c) for testing the lib

#include <stdio.h>
#include "xswrap.h"

int main (){
    int ret = mult(5, 5);
    printf("%d\n", ret);

    speak("hello, world!");

    unsigned char* list = arr(); 

    int i;

    for (i=0; i<3; i++){
        printf("%d\n", list[i]);
    }

    return 0;
}
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The build/install script (build.sh)

#!/bin/sh
gcc -c -fPIC xswrap.c
gcc -shared -fPIC -Wl,-soname,libxswrap.so -o libxswrap.so xswrap.o -l
+c
sudo cp libxswrap.so /usr/lib
sudo cp xswrap.h /usr/local/include
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Done!

The library and its header file are both now put into directories in your PATH.

To compile the test program:

gcc -o test main.c -lxswrap
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...run it:

./test
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You're now ready to wrap the library using Perl and XS per the OP.