Can you give some example of the actual file, e.g. copy a few lines and paste it here in a reply, enclosed in <code></code> tags. Is it a comma-separated file? A spreadsheet of some kind (as the earlier replier inferred)?

I think it's like this:

1/2/2007, 1, 4, 5, 6
1/3/2007, 2, 5, 7, 10
1/5/2007, 5, 6, 8, 11
[download]

And I think the logic is that the date of each line needs to be stripped, and the subsequent entries need to be "subtracted" or hyphen-separated from those in the same column of the previous line. Maybe something like this works:

my @previous;
while(<>){
    chomp;
    my @current = split /,/, $_;
    if ( not @previous ) {
        print join ',', @current[ 1 .. $#current ];
        print "\n";
    }
    else {
        for my $i ( 1 .. ( $#current - 1 ) ) {
            print $current[$i], '-', $previous[$i], ',';
        }
        print $current[-1], '-', $previous[-1], "\n";
        @previous = @current;
    }
}
[download]

Note: not tested the above, but something like this is what I would do.

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