@a = sort @a is done in place before 5.10

>perl580\bin\perl -MO=Concise -e"@a = sort @a" 2>&1 | find "sort"
7           <@> sort lK ->8

>perl588\bin\perl -MO=Concise -e"@a = sort @a" 2>&1 | find "sort"
7           <@> sort lK/INPLACE ->8

>perl5100\bin\perl -MO=Concise -e"@a = sort @a" 2>&1 | find "sort"
7           <@> sort lK/INPLACE ->8
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I don't have 5.8.1 to 5.8.7, so let's consult the perldeltas.

perl584delta:

In place sort optimised (eg @a = sort @a)

But it was buggy in 5.8.4. perl585delta:

The in-place sort optimisation introduced in 5.8.4 had a bug. For example, in code such as @a = sort ($b, @a), the result would omit the value $b. This is now fixed.

It's the same mechanism that sort uses for in-place sorting in 5.10. I've thought about patching List::Util::shuffle() in the same way.

I thought what you meant was that a subroutine can detect its being called in void context? But if I am not mistaken, 5.10's in-place sorting does not happen in void context:

% perl5.10.0 -lwe '@a = (2,1); sort @a; print @a'
Useless use of sort in void context at -e line 1.
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As I understand it, perl (5.10) will detect that in @a = sort @a, the destination array is the same as the source array, so it uses a more efficient algorithm (but it's still in list context).

You're right. It is implemented that way. I should really get around to using these features, not just relying on half remembered discussion of what 5.10 can do.

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