>perl -MO=Concise -e"$a=1"
6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
5     <2> sassign vKS/2 ->6                 <--
3        <$> const[IV 1] s ->4
-        <1> ex-rv2sv sKRM*/1 ->5
4           <#> gvsv[*a] s ->5
-e syntax OK

>perl -MO=Concise -e"($a)=1"
8  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
7     <2> aassign[t2] vKS ->8               <--
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[IV 1] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
-           <1> ex-rv2sv sKPRM*/1 ->-
6              <#> gvsv[*a] s ->7
-e syntax OK