Ran a quick test on my phone:

Looks like the typeglob˛ is looked up and the reference is stored in the OP tree. See \&main::t in #4.

That's why further manipulation will produce different results if the original typeglob isn't targeted. °

(Concise is able to backengineer the original name or is doing other introspection)

~ $ perl -MO=Concise  -e'sub t {}; t()'

6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter v ->2
2     <;> nextstate(main 3 -e:1) v:{ ->3
5     <1> entersub vKS ->6
-        <1> ex-list K ->5
3           <0> pushmark s ->4
-           <1> ex-rv2cv sK/1 ->-
4              <#> gv[IV \&main::t] s ->5
-e syntax OK
[download]

For comparison, that's what's happening when the sub is unknown at compile time

gv[*t] which most likely means "look up the symbol t at runtime"...

~ $ perl -MO=Concise  -e't();'                       
6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter v ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
5     <1> entersub[t2] vKS/TARG ->6
-        <1> ex-list K ->5
3           <0> pushmark s ->4
-           <1> ex-rv2cv sK/1 ->-
4              <#> gv[*t] s/EARLYCV ->5
-e syntax OK
~ $
[download]

°) especially deleting the entry in the namespace prevents the compiler to find the old typeglob.

˛) or is it only the CODE slot? How reliable is concise here???