Your mistake was, that you prepended to filename and afterwards your regex wouldn't match anymore. Here's a fix:

my @files = ();
my $dir = $rootPath . $input->param('path');
$dir .= '/' unless $dir =~ m#/$#;

opendir (DIR, $dir) or die $!;
@files = grep { -f } map { "$dir$_" } grep { !m#^\.# } readdir (DIR);
closedir (DIR);
[download]

That's the working version of your code. But all hidden files (on *n?x, that is) are removed, too. If you want them to, all you have to do is remove that regex. "." and ".." will get filtered out as well since they are directories, too. At least on my box it worked all fine. That one line should then look like this:

@files = grep { -f } map { "$dir$_" } readdir (DIR);
[download]

Regards,
-octo