my $hash = {};

do_something( $hash );
print Dumper( $hash );

sub do_something
{
    my $hash = shift;

    $hash = { a => 'alpha',
              b => 'beta',
            };

    print Dumper( $hash );
}

##</code><code>##

{
  'a' => 'alpha',
  'b' => 'beta'
}
{}

##</code><code>##

do_something_else( $hash );
print Dumper( $hash );

sub do_something_else
{
    my $hash = shift;

    $hash->{g} = 'gamma';
    $hash->{d} = 'delta';

    print Dumper( $hash );
}

##</code><code>##

{
  'd' => 'delta',
  'g' => 'gamma'
}
{
  'd' => 'delta',
  'g' => 'gamma'
}