sub do_something {
    my ( $normal_arg1, $normal_arg2, $optional_hashref ) = @_;
    my $hashref = $optional_hashref ||  { };
    #
    # ...time passes...
    #
    $hashref->{ return1 } = "foo";
    $hashref->{ return2 } = "bar";
    return $hashref;
}

my %hash;

my %newhash = do_something( $this, $that );
do_something( $this, $that, \%hash );