Just be aware that exists($h{a}->{b}) will autovivify the first hash key component ({a}):

use strict;
use warnings;
my %h;

unless (exists($h{foo}{bar})) {
  print "\$h{foo}{bar} exists: no\n";
  print "\$h{foo} exists: ", exists($h{foo})?'yes':'no', "\n";
}

#outputs
#  $h{foo}{bar} exists: no
#  $h{foo} exists: yes
[download]

If you want to be extra careful not to create hash keys, you'll have to use if (exists($h{a} and exists($h{a}->{b}) instead. and (or &&) short circuits so you'll never reach the second test if the first part of the key is non-existent.

Best, beth