use strict;  # ---> IMPORTANT

my $dir = "toto";
my $file;

sub openNewDir {
    my $dir = shift;
    ### my $dh;
    opendir ($dir, $dir);  # ---> MODIFICATION
    while ($file = readdir ($dir)) {      # ---> MODIFICATION here too
        next if (($file eq '.') || ($file eq '..'));
        if (-d "$dir/$file") { openNewDir("$dir/$file"); } 
        else {
            # Do something with the file
            print "($file)\n";
        }
    }
    close ($dir);  # ---> MODIFICATION here too
}
openNewDir ($dir);