Just use File::Copy:

use File::Copy;

opendir(DIR,$file);
my @files = readdir(DIR);
close(DIR);

for my $file (@files) {
    if($file =~ /\.HTML?$/) {
        my $new_file = lc $file;
        move($file, $new_file) or
           die "Can't move file: $!";
    }
}
[download]

Update: Fixed incorrect Perl construct. Thanks to gav^ for noticing it.

--
Ilya Martynov (http://martynov.org/)