the typeglob is created, but it doesn't contain anything.

It's not empty. It caches the SUPER(T2::s1) ⇒ T1::s1 relation.

{ package T1;              sub s1 { }                  }
{ package T2; use base T1;                             }
{ package T3; use base T2; sub s1 { shift->SUPER::s1 } }

use Devel::Peek qw( Dump );

{
  warn("T1::s1: ", \&T1::s1, "\n");
  warn("T2::s1 (pre):\n"); Dump(*T2::s1);
  T3->s1;
  warn("T2::s1 (post):\n"); Dump(*T2::s1);
}
[download]

>perl script.pl 2>&1 | findstr ": CODE CV"
T1::s1: CODE(0x183180c)
T2::s1 (pre):
    CV = 0x0
    CVGEN = 0x0
T2::s1 (post):
    CV = 0x183180c
    CVGEN = 0x91
[download]

SUPER doesn't need to be explicitly mentioned.

{ package T1; sub s1 { }   }
{ package T2; use base T1; }

use Devel::Peek qw( Dump );

{
  warn("T1::s1: ", \&T1::s1, "\n");
  warn("T2::s1 (pre):\n"); Dump(*T2::s1);
  T2->s1;
  warn("T2::s1 (post):\n"); Dump(*T2::s1);
}
[download]

>perl script.pl 2>&1 | findstr ": CODE CV"
T1::s1: CODE(0x183180c)
T2::s1 (pre):
    CV = 0x0
    CVGEN = 0x0
T2::s1 (post):
    CV = 0x183180c
    CVGEN = 0x8e
[download]