Data::Dumper

to visualize a complex data structure rapidly is to dump it.

use Data::Dumper;

my %total_bytes;
while (<>) {
  my ($source, $destination, $bytes) = split;
  $total_bytes{$source}{$destination} += $bytes;
}

print Dumper(\%total_bytes);

myhost% perl bytecounts2 <bytecounts-in
$VAR1 = {
          'thurston.howell.hut' => {
                                     'lovey.howell.hut' => 1250
                                   },
          'ginger.girl.hut' => {
                                 'maryann.girl.hut' => 199,
                                 'professor.hut' => 1218
                               },
          'professor.hut' => {
                               'gilligan.crew.hut' => 1250,
                               'lovey.howell.hut' => 1360
                             }
        };