I was just asking myself exactly that question. Turns out grep requires lvalues, but sort doesn't. 

>perl -MO=Concise -e"@b = grep f, @h{@a}"
...
a                 <@> hslice lKM ->b
...

>perl -MO=Concise -e"@b = sort @h{@a}"
...
a              <@> hslice lK ->b
...

[download]

Notice the "M" for "Modifiable" in the former.

So that begs the question: Why does grep require lvalues? The difference is: 

>perl -e"@a = grep { $_='!'; 0 } @h{qw(i j)};"

>perl -e"@a = sort { $a='!'; 0 } @h{qw(i j)};"
Modification of a read-only value attempted at -e line 1.

[download]

But as far as I can tell, there is no good reason.

In reply to Re^3: Accidentally creating hash elements with grep { defined } on a hash slice by ikegami
in thread Accidentally creating hash elements with grep { defined } on a hash slice by ferment

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