You can't write to the beginning of a file without overwriting data that is already there. Provided the file is not to big this is the easiest (but not the only) way to do it:

my ( @data, $cnt);
while(<DATA>) {
    if ( $cnt++ % 2 ) {
        push @data, $_;
    }
    else {
        unshift @data, $_;
    }
}

# now open new file and just do (to the FILEHANDLE not STDOUT)
print STDOUT @data;

__DATA__
1
2
3
4
5
6
7
8
9
10
[download]

This will rearrage the file to look like:

9
7
5
3
1
2
4
6
8
10
[download]

Another option is to write one half of the data to one file and the other half (alternate lines) to another temp file. As you can see the even lines will be in the right order. The odd lines need reversing. To generate the output file use File::ReadBackwards on the odd line file to pring it out in reverse and then just print out the even lines. This is the way to do it if the file is too big to fit in an array in memory, otherwise I would not bother. You could also use Tie::File and probably a few other things as well.

cheers

tachyon

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