in reply to Why does perl expression in empty nested hash create parents

Sadly, it's a feature, but it's the intermediate keys you are creating. Try this:
print "1st run\n" if exists $a{a}; print "2nd run\n" if exists $a{a}{b}{c}; print "3rd run\n" if exists $a{a}; # prints 3rd run
$a{a}{b} is created in order to test the existence of $a{a}{b}{c}. From perldoc -f exists:
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name: if (exists $ref->{A}->{B}->{$key}) { } if (exists $hash{A}{B}{$key}) { } if (exists $ref->{A}->{B}->[$ix]) { } if (exists $hash{A}{B}[$ix]) { } if (exists &{$ref->{A}{B}{$key}}) { } Although the mostly deeply nested array or hash will not spring into existence just because its existence was tested, any intervening ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring into existence due to the existence test for the $key element above. This happens anywhere the arrow operator is used, including even here: undef $ref; if (exists $ref->{"Some key"}) { } print $ref; # prints HASH(0x80d3d5c) This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a future release.
But you can be smart about this. You can test for $a{a}. If it doesn't exist, any and all subkeys certainly won't exist.