I'm curious by what criteria you judge that it "should have been." Do you mean that it should be there as a matter of good defensive programming style?

Nothing to do with style, but with correctness. Stuff like:

    while (($line = <>) && $line =~ /\S/) { ... }
    while (($line1 = <>) && ($line2 = <>)) { ... }
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Also, the standard way of translating a while to a for(;;) loop won't work:

    for (;$line = <>;) { ... }
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And even:

    next unless $line = <>;
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They are all cases where a defined test should be used, because otherwise you may get unwanted effects.

Abigail