fixdir('d');

sub fixdir {
    my $dir = shift;
    opendir my $DH, $dir or die "$dir: $!";
    while (my $f = readdir $DH) {
        next if grep $_ eq $f, qw/. ../;
        (my $new = $f) =~ s/[^a-zA-Z0-9_.]/_/g;
        die "$f: $new already exists.\n" if -e "$dir/$new" and $new ne $f;
        print STDERR "Renaming: $f -> $new\n";
        rename "$dir/$f", "$dir/$new";
        fixdir("$dir/$new") if -d "$dir/$new";
    }
}