use File::Find;
finddepth {
  wanted => sub {
    return if /^\./;
    my $new = lc $_;
    return if $new eq $_ or -e $new;
    rename $_, $new or warn "Cannot rename $File::Find::name to $File::Find::dir/$new: $!";
  },
  follow => 1,
}, $ARGV[0] || ".";