Clutter::Cogl::Handle is an opaque data type that is used to store a handle to a GL or GLES resource. A handle can point to a texture, or a shader program, or an offscreen buffer.
The nature and contents of the handle are completely shielded from the Perl developer; a handle can only be used with the Clutter::Cogl functions.
Clutter::Cogl is an abstraction API over GL and GLES, and it is used internally by Clutter to allow portability between platforms.
The Clutter::Cogl API is low-level and it is meant to be used only when creating new Clutter::Actor classes.
Clutter::Cogl tries to provide an API that is nicer and more understandable than the raw OpenGL API (as exposed, for instance, by the Perl OpenGL wrapper module).