Typical data reduction using references. Get input file (coconet.dat) from http://www.intermediateperl.com/downloads_page/.

net_stats.pl

#!/usr/bin/perl
use strict;
use warnings;

my $all = "**all machines**";
my %total_bytes;
while (<>) {
    next if /^#/;
    my ( $src, $dst, $bytes ) = split;
    $total_bytes{$src}{$dst} += $bytes;
    $total_bytes{$src}{$all} += $bytes;
}

my @sources =
  sort { $total_bytes{$b}{$all} <=> $total_bytes{$a}{$all} } keys %total_bytes;

for my $src (@sources) {
    my @destinations =
      sort { $total_bytes{$src}{$b} <=> $total_bytes{$src}{$a} }
      keys %{ $total_bytes{$src} };
    print "$src sent $total_bytes{$src}{$all} total bytes\n";
    for my $dst (@destinations) {
        next if $dst eq $all;
        print "\t=> $dst: $total_bytes{$src}{$dst} bytes\n";
    }
}