I have to admit that I am not a specialist on the Perl internals (more precisely I do not know anything about it), so my guess is the following:

If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the foreach loop index variable is an implicit alias for each item in the list that you're looping over.

from perlsyn means that for anything that can be modified (i.e. an lvalue) in the loop's body, Perl must remember where it is stored. When accessing or modifying it, the tie mechanism is invoked. So this version

tie @ary, "My::Class", "some", "el", "ems";
for (@ary) {
   some_code($_);
}
[download]

must have worked ever since tie exists. Using sub wrapper : lvalue {...; return @x} is only turning the result from the sub to be an lvalue (according to the docs since Perl 5.6).

Whether or not this is what is really happening I do not know.

Update: reversed lvalues seem to be fine as well:

my @arr1 = (0) x 4;
my $n = 0;
for( reverse @arr1 ) {
    $_ += $n++;
}
print "@arr1\n";

__END__
Output
3 2 1 0
[download]