Hmm. Your code isn't even remotely valid perl. Perhaps you should look at the documentation a bit more?

Here is something that would work

opendir(DIR, $dir) or die "Cannot open directory '$dir': $!";
my @files = readdir(DIR);
close(DIR);

foreach my $file (@files) {
    if ($file =~ /\.HTML?$/i) {
        system("rename $file \L$file\E");
    }
}
[download]

gav^