Ah, I see what you mean. You're wrong about it dying, though. It's only warning.
I don't find it unusual since I've previously heard that { $a cmp $b }, { $a <=> $b }, { $b cmp $a } and { $b <=> $a } are handled specially to avoid having to call a Perl sub repeatedly.
>perl -MO=Concise -e"@a = sort { $a <=> $b } @a"
8 <@> leave[1 ref] vKP/REFC ->(end)
1 <0> enter ->2
2 <;> nextstate(main 2 -e:1) v ->3
- <1> ex-aassign vKS/64 ->8
- <1> ex-list lK ->-
3 <0> pushmark s ->4
7 <@> sort lK/INPLACE,NUM ->8
4 <0> pushmark s ->5
6 <1> rv2av[t7] lKRM*/1 ->7
5 <#> gv[*a] s ->6
- <1> ex-list lK ->-
- <0> ex-pushmark s ->-
- <1> ex-rv2av lKRM*/1 ->-
- <0> ex-gv s ->-
-e syntax OK
>perl -MO=Concise -e"@a = sort { 1; $a <=> $b } @a"
8 <@> leave[1 ref] vKP/REFC ->(end)
1 <0> enter ->2
2 <;> nextstate(main 2 -e:1) v ->3
- <1> ex-aassign vKS/64 ->8
- <1> ex-list lK ->-
3 <0> pushmark s ->4
7 <@> sort lKS*/INPLACE ->8
4 <0> pushmark s ->5
- <1> null sK/1 ->5
- <1> ex-leave sKP ->-
- <0> enter s ->-
- <;> nextstate(main 1 -e:1) v ->-
- <0> ex-const v ->-
- <;> nextstate(main 1 -e:1) v ->-
- <2> ncmp[t5] sK/2 ->(end)
- <1> ex-rv2sv sK/1 ->-
- <#> gvsv[*a] s ->-
- <1> ex-rv2sv sK/1 ->-
- <#> gvsv[*b] s ->-
6 <1> rv2av[t7] lKRM*/1 ->7
5 <#> gv[*a] s ->6
- <1> ex-list lK ->-
- <0> ex-pushmark s ->-
- <1> ex-rv2av lKRM*/1 ->-
- <0> ex-gv s ->-
-e syntax OK
While I don't find it unusual, I do think the difference in behaviour is a bug.