is magic, please check the Perldoc in perlsyn.
> If the condition expression of a while statement is based on any of a group of iterative expression types then it gets some magic treatment. The affected iterative expression types are readline, the<FILEHANDLE> input operator,readdir, glob, the <PATTERN>globbing operator, and each. If the condition expression is one of these expression types, then the value yielded by the iterative operator will be implicitly assigned to $_. If the condition expression is one of these expression types or an explicit assignment of one of them to a scalar, then the condition actually tests for definedness of the expression's value, not for its regular truth value.
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