use strict;
use warnings;

my @data;

<DATA>; # Skip header.

while (<DATA>) {
   chomp;
   push(@data, [ map { 0+$_ } split(/\t/, $_) ]);
}

# Sort by "In".
my @in_sorted = sort { $a->[1] <=> $b->[1] } @data;

# Sort by "Out".
my @out_sorted = sort { $a->[2] <=> $b->[2] } @data;

# Sum "In" and "Out" by "Day".
my %in_day;
my %out_day;
foreach (@data) {
   my $day = $_->[3];

   # Avoid warnings.
   $in_day{$day}  ||= 0;
   $out_day{$day} ||= 0;

   $in_day{$day}  += $_->[1];  # "In"
   $out_day{$day} += $_->[2];  # "Out"
}

__DATA__
ID	In	Out	Day
1	5	2	1
2	4	9	2
3	3	3	2
4	6	7	3
5	5	0	5
6	7	9	3
7	8	9	4
8	6	6	4