Just to rephrase this a little. Scalar variables are containers that hold a value. When you assign to a scalar you throw away the value previously contained and overwrite it with a new value.

References are a scalar value that point at (or refer to) other containers. This should be distinguished from an alias which is just a different textual name for the same container. For instance if you modify the first sub:

sub foo {
  for my $ref ($_[0]) {
    $ref = { all => new };
  }
}
my $hash={};
print $hash,"\n";
foo($hash);
print $hash,"\n";
[download]

$ref is now an alias to $_[0] which is itself automagically an alias to whatever scalar was passed into the subroutine which in this case is the scalar $hash. In this case assigning to $ref inside of the sub is the same as assigning to $hash outside of the sub.

On rereading this it may just cause more confusion, if so I apologise.