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

Why does grep require lvalues?

So you can suprise people with what this does:

perl -e "@in = (1..5); @out = grep { $_++ } @in; print qq{@in}"
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But as far as I can tell, there is no good reason.

Oh - a good reason? Sorry, haven't got one of them. Move along please, nothing to see here.

--
.sig : File not found.

True, but it's a bad example since @in naturally returns lvalues.

>perl -le" @in = (1..5); @out = sort { $a=$b='!'; 0 } @in; print qq{@i
+n}"
! ! ! ! !
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Note the absence of the "M" flag.

>perl -MO=Concise -le" @in = (1..5); @out = sort { $a=$b='!'; 0 } @in;
+ print qq{@in}"
...
e              <1> rv2av[t11] lK/1 ->f
...
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It will affect

pp_pos       - Adds lvalue magic
pp_substr    - Adds lvalue magic
pp_vec       - Adds lvalue magic
pp_aelem     - Extends array
pp_aelemfast - Extends array
pp_aslice    - Extends array
pp_helem     - Extends hash
pp_hslice    - Extends hash
pp_padsv     - No effect as far as I can tell
pp_rv2sv     - ???
pp_rv2av     - ???
pp_rv2gv     - ???
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Thanks both for your replies. I understand the problem now :)