I do not see the point of modifying $i when you reach the end of the loop (and it is a bit awkward and confusing), but if you do it, it works:

$ perl -e 'my $v = 1;
> for ($i = 0; $i <=100; $i++) {
>     $i = 2* $i if defined $v and $i == 100;
> }
> print $i;'
201
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So, I would assume that, probably, $v is not defined in your case.

Having said that, you probably don't really want to use a C-style for loop for that, but rather a while loop.