You can use -d, but you have to be sure you're testing the correct pathname. Note that readdir returns unqualified names relative to the directory they're in.

my $dir = "/anydir";
opendir DIR, $dir or die "opendir $dir : $! \n";

for ( readdir DIR ) {
    my $dirent = "$dir/$_";
    if ( -d $dirent ) {
        print "$dirent is a directory\n";
    }
}

closedir DIR;
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