Exists checks to see that the lookup of $var->{$key} is successful, meaning that $key has been used to store a value in the hash prior to the check. Thus we have:

# $user_by_uid{$uid}->{$host}
$var= $user_by_uid{$uid}
$key= $host
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What this means is that if the result of $user_by_uid{$uid} is undef (either by having an explicit undef value or by not being in the %user_by_uid hash at all) then it is autovivified. If there is a hash reference stored in $user_by_uid{$uid} (either intentionally or via autoviv) then the it is checked to see if $host had ever been used as a key.

Tring to reduce this to a one liner: exists checks the the last hash lookup you feed it, not if the full "path" through the data structure exists. This is more obvious if you consider $user_by_id[$uid]->{$host}, obviously exists doesnt operate on arrays, and we get no error, so it means that only ->{$host} part is relevent to exists.

You could always write a simple routine:

sub my_exists {
  my ($hash_ref,$key)= @_;
  return $hash_ref and exists $hash_ref->{$key}
}
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which would not autovivify. Compare it to how the real thing *might* be coded. Im pretty sure this actually wont work due to some subtleties of hash lookups and lvalue arguments but its pretty close to the real thing.

sub my_fake_exists {
  my ($hash_ref,$key)=@_;
  $hash_ref={} if not defined $hash_ref; #fake autoviv
  return exists $hash_ref->{$key}; # do the lookup
}
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