If you look into the optree you'll see a list assignment aassign * happening in the second example instead of an scalar assignment sassign

C:\Windows\system32>perl -MO=Concise -E"substr((  substr( $p, 0, 1 ) =
+ '123' ), 0, 1 ) = 'x';"
d  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 47 -e:1) v:%,{,469764096 ->3
-     <1> ex-sassign vKS/2 ->d
c        <@> substr[t5] vKS/REPL1ST,3 ->-
-           <0> ex-pushmark s ->4
3           <$> const[PV "x"] s ->4
9           <2> sassign sKPRMS*/2 ->a
4              <$> const[PV "123"] s ->5
8              <@> substr[t3] sKRM*/3 ->9
-                 <0> ex-pushmark s ->5
-                 <1> ex-rv2sv sKRM*/1 ->6
5                    <#> gvsv[*p] s ->6
6                 <$> const[IV 0] s ->7
7                 <$> const[IV 1] s ->8
a           <$> const[IV 0] s ->b
b           <$> const[IV 1] s ->c
-e syntax OK

C:\Windows\system32>perl -MO=Concise -E"substr((( substr  $q, 0, 1 ) =
+ '123' ), 0, 1 ) = 'x'"
f  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 47 -e:1) v:%,{,469764096 ->3
-     <1> ex-sassign vKS/2 ->f
e        <@> substr[t6] vKS/REPL1ST,3 ->-
-           <0> ex-pushmark s ->4
3           <$> const[PV "x"] s ->4
b           <2> aassign[t4] sKPRMS* ->c
-              <1> ex-list lK ->6
4                 <0> pushmark s ->5
5                 <$> const[PV "123"] s ->6
-              <1> ex-list lK ->b
6                 <0> pushmark s ->7
a                 <@> substr[t3] sKPRM*/3 ->b
-                    <0> ex-pushmark s ->7
-                    <1> ex-rv2sv sKRM*/1 ->8
7                       <#> gvsv[*q] s ->8
8                    <$> const[IV 0] s ->9
9                    <$> const[IV 1] s ->a
c           <$> const[IV 0] s ->d
d           <$> const[IV 1] s ->e
-e syntax OK
[download]

edit

see ikegami's answer here Re: Why Perl gets confused here?

*) aassign for "array" assignment is one of these misnomers like wantarray , should rather be named lassign

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