sub _create_package {
    ...
    $self->_write_worksheet_files();
}   
sub _write_worksheet_files {
    my $self = shift;
    my $dir  = $self->{_package_dir};

    _mkdir( $dir . '/xl' );
    _mkdir( $dir . '/xl/worksheets' );
    ...
}
sub _mkdir {
    my $dir = shift;

    return if -e $dir;

    my $ret = mkdir( $dir );

    if ( !$ret ) {
        croak "Couldn't create sub directory $dir: $!";
    }
}