format foo =
.

{
    package mypkg;
    *form_foo = *main::foo;
    *form_ref = *form_foo{FORMAT};
}

use Devel::Peek;

Dump(*main::foo);
Dump(*mypkg::form_foo);
Dump(*mypkg::form_ref);

__output__

SV = PVGV(0x80f9c78) at 0x8100de8
...
    FORM = 0x80fd4e4  
SV = PVGV(0x80f9dc0) at 0x8100e24
...
    FORM = 0x80fd4e4  
SV = PVGV(0x8101820) at 0x8100e3c
...
    FORM = 0x0