...
    use MyData;
    $f = new MyData(file => "foo.compressed",encrypt_key =>"");
    ...

package MyData;

sub new
  {
    ...
    my $parser = XML::SAX::ParserFactory->parser(
                            Handler => MyData::Handler->new);

    my $fh = _file_open($filename,$key);
    $parser->parse_file($fh);
    ...
  }

sub _file_open
  {
    my($file_name,$encrypt_key) = @_;
    $encrypt_key = "" if(!defined $encrypt_key);

    # This first bit works OK
    my $compress = "";
    my $compress = 1 if($file_name =~ /\.compressed$/);
    if($encrypt_key eq "")
      {
        return new IO::Zlib($file_name,"rb") if($compress);
        return new IO::File($file_name,"r");
      }
    # This is where I get into trouble
    ?????
  }