Perhaps another monk can explain this behavior.

\&Foo::some_sub autovivifies a sub stub at run-time (just like sub Foo::some_sub; would at compile-time).

$ perl -le'
   print(exists(&Foo::some_sub)?1:0, defined(&Foo::some_sub)?1:0);
   $x=\&Foo::some_sub;
   print(exists(&Foo::some_sub)?1:0, defined(&Foo::some_sub)?1:0);
   eval "sub Foo::some_sub { print \"foo\" }";
   print(exists(&Foo::some_sub)?1:0, defined(&Foo::some_sub)?1:0);
   $x->();
'
00
10
11
foo
[download]

As demonstrated, this permits the reference to be taken to a sub that has yet to be defined.

\&Foo::some_sub creates glob entry at compile-time.

$ perl -le'
   print(exists($Foo::{not_a_sub})?1:0);
   print(exists($Foo::{some_sub})?1:0);
   $x=\&Foo::some_sub;
'
0
1
[download]