It shows up as const AV in the opcode tree.

You can prove the list is flattened once using

I see the 1 & 2, and 4 & 8 as consts, but I'm also seeing what look to me like two loops. Perhaps you can interpret it better?:

c:\test>perl -MO=Concise
for (1..2) {
    for (4..8) {
       say $_++;
    }
 }
^Z
t  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 4 -:1) v:{ ->3
s     <2> leaveloop vK/2 ->t
7        <{> enteriter(next->p last->s redo->8) lKS/8 ->q
-           <0> ex-pushmark s ->3
-           <1> ex-list lK ->6
3              <0> pushmark s ->4
4              <$> const[IV 1] s ->5
5              <$> const[IV 2] s ->6
6           <#> gv[*_] s ->7
-        <1> null vK/1 ->s
r           <|> and(other->8) vK/1 ->s
q              <0> iter s ->r
-              <@> lineseq vKP ->-
8                 <;> nextstate(main 2 -:2) v:{ ->9
o                 <2> leaveloop vK/2 ->p
d                    <{> enteriter(next->l last->o redo->e) lKS/8 ->m
-                       <0> ex-pushmark s ->9
-                       <1> ex-list lK ->c
9                          <0> pushmark s ->a
a                          <$> const[IV 4] s ->b
b                          <$> const[IV 8] s ->c
c                       <#> gv[*_] s ->d
-                    <1> null vK/1 ->o
n                       <|> and(other->e) vK/1 ->o
m                          <0> iter s ->n
-                          <@> lineseq vKP ->-
e                             <;> nextstate(main 1 -:3) v:{ ->f
k                             <1> preinc[t7] vK/1 ->l
j                                <1> entersub[t6] sKRMS/NO(),TARG ->k
f                                   <0> pushmark s ->g
-                                   <1> ex-rv2sv sKM/1 ->h
g                                      <#> gvsv[*_] s ->h
h                                   <$> method_named[PV "say"] s ->i
i                                   <1> rv2cv /NO() ->j
l                             <0> unstack v ->m
p                 <0> unstack v ->q
- syntax OK
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