>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
...

##</code><code>##

>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.