package File::MyFileType;

sub open {
    my ($class, $filename ) = @_;
    ...
    $self->_initialize_headers;
    # initialize headers would write something like
    # Checksum: XXXXXXXXXXXXXX in the first line
    # reserving space for file's header
}

sub write {
   my ($self, $data) = @_;
   $self->_update_checksum($data);  
}

sub close {
   $self->_update_headers;
   # Update header rewinds the file and replaces the
   # XXXXXXXXXX with the accumulated checksum 
   ...
}

1;