In C one of the most common errors to make is an off-by-one error. If you use explicit for loops in Perl, you will make the same error just as often. This is a good reason to use foreach loops whenever possible.

I mention this because your second observation has an off-by-one in it. The odds of an off-by-one can be reduced, though, by:

foreach my $i (0..$#filenames) {
  # ...
}
[download]

since that removes an inequality that people may miswrite. An even cleaner solution is offered by tye at mapcar -- map for more than one list.