In brief, join() is a plain function. It does not evaluate its first argument (or any argument) more than once. In your example, $i is incremented, but after the join function has been called and returns.

I'm pretty sure that the only things that evaluate an argument more than once will accept a BLOCK type argument. For example, sort BLOCK LIST or map BLOCK LIST. Curly braces. A bit of code to be evaluated on each iteration.

my $result = '';
my @list = qw(one two three four five six seven eight);
$result .= (shift @list).':'.(shift @list) while @list;
print $result, $/;
__OUTPUT__
one:twothree:fourfive:sixseven:eight
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