Ken's comments are "spot on".
The second form is far preferable because it is just far more clear!

There is however another "layer" of complexity to this question that you are probably not aware of....

When testing for the existence of some value in a 2D hash, Perl will "autovivify" the first dimension(s) if it(they) doesn't exist already. The code below demonstrates this behavior. This can lead to some side-effects and strange results later.

This statement:

$var = $vxdg{$node}{$disk} //= '';
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creates the entire multi-dimensional key and assigns it the value of '' if it wasn't already defined. This '//=' operator is specific to Perl. It is like ||= except that it tests for "defined-ness" instead of "truth-ness". Whether or not this is a "good thing" or "not" depends upon the application.

I'm just suggesting that you should be aware of what happens when testing for existence or "defined-ness" of multi-dimensional hash array values.

#!/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: ""
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