Demonstrating the logical defined-or operator

usage.pl

#!/usr/bin/perl                                                                 

use strict;                                                                     
use warnings;                                                                   

my $NewUsage = 150;                                                             
my %Usage = ();                                                                 

foreach my $mac ('ABC', 'DEF', 'ABC', 'DEF', 'GHI') {                           
    $Usage{$mac} = $Usage{$mac} // 0;                                           
    $Usage{$mac} += $NewUsage;                                                  
}                                                                               

use Data::Dumper;                                                               
print Dumper(\%Usage);

__DATA__
$VAR1 = {
          'ABC' => 300,
          'DEF' => 300,
          'GHI' => 150
        };