Question

{%{$self->param}}

It does hash expand, and then create another hash reference.

But isn’t {%{$self->param}} the same as $self->param? Why does the code bother doing the trick?

Answer 1

It copies the hash. Consider the following snippet:

use Data::Dumper;

my $foo = { a => 1, ar => [1] };
my $bar = {%$foo};

$bar->{b} = 2;
push @{$bar->{ar}}, 4;

print Dumper $foo;
print Dumper $bar;

It prints

$VAR1 = {
          'a' => 1,
          'ar' => [
                    1,
                    4
                  ]
        };
$VAR1 = {
          'a' => 1,
          'b' => 2,
          'ar' => [
                    1,
                    4
                  ]
        };

So you can see that the copy is shallow: Scalars are copied over, even if they are references. The referenced objects are the same (in this example the array referenced by ar).