Sorry for the lack of code in my original question; it is not so easy to demonstrate in a code snippet, and impossible to show the data, but here goes... 
    open(IN,  " < $infile" );

    my $si=8192;  # the number of entries is known

    my %matrix;

    while(my $c<$si) {

        read(IN, my $bin, 4);
        $data = unpack('N', $bin);          
            
        my $d = Unshuffle($c);
          
        $matrix{$d} += $data;
            
        $c++;
    }

    close( IN );

    open(OUT, " >$outfile" );

    foreach my $d (sort {$a <=> $b} (keys %matrix)) {
        print (OUT pack('V', $matrix{$d}));
    }

    close( OUT );

[download]
The subroutine Unshuffle() simply calculates a new position in the output matrix for each entry. The above works until I try to scale the data, e.g.  $matrix{$d} += $data/3; In which case the output is scrambled. BTW, null opperations like:  $matrix{$d} += $data/1;  $matrix{$d} += $data+0; still work. This, I suspect, explains the fact that the script also fails if more than one of data points added to make $matrix{$d} is non-zero. I guess that my number is now too long for the "V" template in pack. Yes, the template must be 'V', otherwise a downstream program spits the dummy. thanks again

In reply to Re: Data munging - in, out and something in between by Anonymous Monk
in thread Data munging - in, out and something in between by Anonymous Monk

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