I am looking for a short form for...

if (exists $vxdgs{$node}{$disk}) {
 my $vxdg=$vxdgs{$node}{$disk};
}
else {
 my $vxdg="";
}
[download]

Well, assuming %{$vxdgs}{$node} isn't a tied hash with side-effects, and $vxdgs{$node}{$disk} isn't a tied scalar with side-effects, your code is a noop. Both blocks assign something to a scalar variable, with both scalars going out of scope directly after the assignment.