Actually, auto-vivification of hash entries doesn't happen unless you do something like take a reference to the hash entry... for example by referring to something more deeply nested in the non-existent hash entry:

#!/usr/bin/perl -l
my %hash;

my $foo = $hash{'foo'};
print exists $hash{'foo'} ? "yes" : "no";  # prints "no"

my $bar = $hash{'bar'}->{'xxx'};
print exists $hash{'bar'} ? "yes" : "no";  # prints "yes"

my $baz = \$hash{'baz'};
print exists $hash{'baz'} ? "yes" : "no";  # prints "yes"
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