use Data::Dumper;

my %x = ("a"  => "red");
my %y = ("b"  => "green");

modifyHash( \(%x, %y) );

sub modifyHash
{
  my ($x, $y) = @_;
  $x->{a} = "circle";
  $x->{b} = "square";
}

print Dumper ( \(%x, %y) );