$var = $vxdg{$node}{$disk} //= '';

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %vxdg;
my $var;

my ($node,$disk) = (3, "C:");
print "node= $node disk=$disk\n";

print "orignal hash is:", Dumper \%vxdg;
print "\$var is: \"", (defined $var) ? $var : "undefined", "\"","\n"; 

$var = exists $vxdg{$node}{$disk} ? $vxdg{$node}{$disk} : '';
print "the hash is now...after checking for",'$vxdg{$node}{$disk}:', 
       "\n", Dumper \%vxdg;
print "\$var is: \"", (defined $var) ? $var : "undefined", "\"","\n"; 

print "\n***Starting over..***\n";
%vxdg=();
$var=undef;

print "orignal hash is:", Dumper \%vxdg;
$var = $vxdg{$node}{$disk} //= '';
print "the hash is now...:", "\n", Dumper \%vxdg;
print "\$var is: \"", (defined $var) ? $var : "undefined", "\"","\n"; 

__END__
node= 3 disk=C:
orignal hash is:$VAR1 = {};
$var is: "undefined"
the hash is now...after checking for$vxdg{$node}{$disk}:
$VAR1 = {
          '3' => {}
        };
$var is: ""

***Starting over..***
orignal hash is:$VAR1 = {};
the hash is now...:
$VAR1 = {
          '3' => {
                   'C:' => ''
                 }
        };
$var is: ""