that doesn't mean the magic variable should not be provided at all, it just means it should be read-only.

That looks like inconsistency to me.

sub f1
{
my ($x, $y) = @_;
z($x, $y);
}
[download]

and

sub f2
{
z($_[0], $_[1]);
}
[download]

Above two examples are not identical. Later one is something low-level, and it's just asking for trouble (for example in cases like f2($1), f2($.) or f2($x, $x) when z() modifies arguments ).
(NOTE: another common point of view is that caller of such functions is asking for trouble)

Example:

sub z
{
$_[0]++;
print $_[1]
}
sub f1
{
my ($x, $y) = @_;
z($x, $y);
}
my $x = 4;
f1($x, $x);
[download]

prints 4

sub z
{
$_[0]++;
print $_[1]
}
sub f2
{
z($_[0], $_[1]);
}
my $x = 4;
f2($x, $x);
[download]

prints 5

So IMHO you should not use @_ just because it's shortest. Use it when you know what you're doing.

And now you suggest add another %_ which behaves completely different way.

Also making %_ readonly won't help much in cases when you pass magic variable as argument (example: RT#54728).