Do you have any documentation for that? The sort manpage doesn't mention any such thing.

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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See: perl584delta