Consider this:

use Data::Dumper;
use List::Util qw(first);

my @x = ({"aaa" => 100});

[grep { $_->{"aaa"} < 4 } @x]->[0] = 'foo';
(first { $_->{"aaa"} < 4 } @x) = 'foo';
[download]

Line 6 (grep) executes fine. Line 7 (first) dies with:

Can't modify non-lvalue subroutine call of &List::Util::first in list assignment at foo.pl line 7, near "'foo';"

You're putting the results of your grep into an arrayref. That arrayref and its elements are lvalues—they can be assigned to—so autovivification can happen to them.