$hash_ref = modifyHash(\%my_hash);
%my_hash = %{$hash_ref}; 
# hash is now modified

sub modifyHash()
{
  my $hash_ref = shift;
  my %sub_hash = %{$hash_ref};

  # Do something to the hash 
  $sub_hash{'test'}=1;

  return (\%sub_hash);

}