%{$hash{'wrong'}}; implies $hash{'wrong'} is a hash ref. Since $hash{'wrong'} is not defined, perl creates an anonymous hash for you and places a reference to it in $hash{'wrong'}. This is called Autovivification.

To avoid it here, do
print($hash{'wrong'} ? () : keys(%{$hash{'wrong'}}), "\n");
or
print(keys(%{$hash{'wrong'}}), "\n") if $hash{'wrong'};
depending if you want a blank line or no line at all.