When used in a numeric context (like the above), $i will automatically take on the value of 0. When used in a string context, an undef value will take on the value of the empty string.

However, I'm pretty sure that in some versions of Perl you get "unitialized value" warnings (when -w is turned on) when using an undef value in concatenation or in a numeric expression.

This is somewhat confirmed by this thread on clpm.

Also, note that if you have -w turned on, you'll get an "unitialized value" warning when you compare $i against 5 the first time through your while loop. See:

    % perl -w
    my $i;
    while ($i<5) {
    $i++;
    }
    <Ctrl-D>
    Use of uninitialized value at - line 2.
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Personally I find this rather annoying. :)