yes, but when $string is undefined, it says...

It's only because you are using a version older than Perl 5.12, because, as ikegami said in Re^3: Checking string emptiness, that behavior was changed in 5.12.

If you are going to choose to use something as old as Perl 5.8, you need to understand that the results you see are not necessarily going to match those that are seen by the majority of the users, because of bugfixes and improvements over the last two decades.

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edit: removed unhelpful sentence