Basic Idea

An identifier of which the type is the rvalue reference is still a lvalue, since the identifier can be identified.
Note that the value which can be identified must be a lvalue([Stroustrup 2013] / Part II Basic Facilities / 6 Types and Declarations / 6.4.1 Lvalues and Rvalues).

Example without Perfect Forwarding

For example, the following code will fail when compiled.

#include <utility>

void foo(int &&i)
{
    
}

int main()
{
    int i = 1;
    int &&r = std::move(i);

    foo(r);

    return 0;
}

main.cpp: In function ‘int main()’:
main.cpp:13:9: error: cannot bind rvalue reference of type ‘int&&’ to lvalue of type ‘int’
     foo(r);
         ^
main.cpp:3:6: note:   initializing argument 1 of ‘void foo(int&&)’
 void foo(int &&i)

Example with Perfect Forwarding

The "std::forward" can be used to fix the compilation error.

#include <utility>

void foo(int &&i)
{
    
}

int main()
{
    int i = 1;
    int &&r = std::move(i);

    foo(std::forward<decltype(r)>(r));

    return 0;
}

Reference

[Stroustrup 2013] Bjarne Stroustrup. "The C++ Programming Language Fourth Edition." Pearson Education 2013.