How is it possible that $atexit holds 2 objects which are both destroyed at scopes end?

Because there are 2 $atexit variables - a masked one, and a masking one created later.

This is all to fullfill what hath been written:

"my" variable $var masks earlier declaration in same scope
[download]

Since it masks that variable - and does not replace it since this would mean destroying the current my variable of that name, the scope of the masked variable ends when the masking ends - because the scope of the masking variable ends - which is: at scope end.

IMHO this is not an implementation specific detail, but part of the the design of my and perl scopes, thus part of the language.