my $command = "tar --tape-length $TAPE_LENGTH -cf $destination $source";
    my ( $input, $output );
    open3( $input, $output, '', $command );

    while ( my $line = <$output> ) {
        print "from tar: '$line'\n";
        if ( $line =~ m/Prepare volume/ ) {
            # need to change tapes
            ...
        }
    }