while (my $line = <Input>) {
    $line =~ s{(z0)}{&#64257;$1}g;
#     $line =~ s{(?<=^)\n\Z}{&#64257;\n}g;
    if ($line =~ /^&#64257;/) {       # line starts with a bracket
        if (@unsorted_input) {  # if array has entries
#             my @sorted = sort two_way_sort @unsorted_input;
            print @unsorted_input;
            undef @unsorted_input;
        }
        print {Output} $line;            # print the header line
    }
    else {                       # line is an entry line
        my @sorted = sort two_way_sort @unsorted_input;
        push @unsorted_input, $line;
    }
}


foreach my $line (@unsorted_input) {
    print Output $line;
}