AFAIK, it is not officially documented in any place, but you can see it's true dumping the OP tree:

salva@vespa:/tmp$ perl -MO=Concise -e '@i = sort @j'
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
b     <2> aassign[t5] vKS ->c
-        <1> ex-list lK ->8
3           <0> pushmark s ->4
7           <@> sort lK ->8
4              <0> pushmark s ->5
6              <1> rv2av[t4] lK/1 ->7
5                 <#> gv[*j] s ->6
-        <1> ex-list lK ->b
8           <0> pushmark s ->9
a           <1> rv2av[t2] lKRM*/1 ->b
9              <#> gv[*i] s ->a
-e syntax OK

salva@vespa:/tmp$ perl -MO=Concise -e '@i = sort @i'
8  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
-     <1> ex-aassign vKS/64 ->8
-        <1> ex-list lK ->-
3           <0> pushmark s ->4
7           <@> sort lK/INPLACE ->8
4              <0> pushmark s ->5
6              <1> rv2av[t4] lKRM*/1 ->7
5                 <#> gv[*i] s ->6
-        <1> ex-list lK ->-
-           <0> ex-pushmark s ->-
-           <1> ex-rv2av lKRM*/1 ->-
-              <0> ex-gv s ->-
-e syntax OK
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