Thank you everyone for answers, now it became more clear. As I understand, every statement in Perl "returns" a value -- rather, it can be evaluated if program flow requires it. The bare "next;" is a statement. What it evaluates to is never necessary to know. The "foo unless bar;" is a statement, too. If "bar" is false, it obviously evaluates to "foo". But if "bar" is true, this whole statement, unexpectedly to me as of yesterday, evaluates to "bar":

>perl -wE "$x=33; say do{42 unless $x}"
33
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The (wrong) intuition was that result would be something like "undef". But, if I get RonW's explanation right, the "evaluator" is more straightforward and "primitive" -- it doesn't try to "understand" statement as a whole, but whatever subexpression was evaluated last is taken as result for a whole.