Difference between , and ;?

In this example,

#include <iostream>    
#include <cstring>    
using namespace std;    

class Libro {    
 char* titulo_; int paginas_;    
 public:    
 Libro() : titulo_(new char[1]), paginas_(0) {*titulo_= 0;}    
 Libro(const char* t, int p) : paginas_(p) {    
  titulo_ = new char[strlen(t) + 1];    
  strcpy(titulo_, t);    
}    
 ~Libro() { delete[] titulo_; }    
 void paginas(int p) { paginas_ = p; }    
 int paginas() const { return paginas_; }    
 char* titulo() const { return titulo_; }    
};    

void mostrar(Libro l) {    
cout << l.titulo() << " tiene " << l.paginas() << " paginas" << endl;    
}    

int main() {    
 Libro l1("Fundamentos de C++", 474), l2("Por Fin: C ISO", 224), l3;    
 l3 = l1;    
 mostrar(l1), mostrar(l2), mostrar(l3);  
}

Despite the copy constructor is not defined and the default copy constructor provide by the compiler is not valid in this case, the execution is correct and It shows the right information in the calls to mostrar(l1), mostrar(l2), mostrar(l3);.

However, if we use mostrar(l1); mostrar(l2); mostrar(l3); instead, we would have the expected error, the last call wouldn't show correctly the last call, because the copy haven't been done properly. Do you know what is the diference between use , and ;? Why this code is working when you use ,?