my $twig = XML::Twig->new();
while (<$fh>) {
    $twig->parse($_);
    # ...
} 

##</code><code>##

while (<$fh>) {
    my $twig = XML::Twig->new($data_struct);
    $twig->parse($_);
    # ...
} 

##</code><code>##

my $twig = XML::Twig->new($data_struct);
while (<$fh>) {
    $twig->parse($_);
    # ...
    my $copy = deep_copy($data_struct);
    $data_struct = clear_structure($data_struct);
}