The parameters in @_ are aliased to what was actually passed, so this works:

sub foo {
   $_[0] = { all => "new" };
}
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but your my $ref = copies the reference, and the copy in $ref is not aliased. This is just like the difference between:

my %x = (foo => 0, bar => 1);

my $z = $x{foo};
$z = 2;
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which doesn't change the original value in %x and

my %x = (foo => 0, bar => 1);

$x{foo} = 2;
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which does.