Your code (slightly modified in a non-significant way) works fine on my NT4 box as

#! perl -w
use Win32::Perms;
sub AddGroupToDir
{
    my ($group, $dir)=@_;
    my $perms = new Win32::Perms($dir) || die "\n$^E\n";
    my $counter = $perms->Get(\ my @list);
    print "\nAccount '$group'\t Path " . $perms->Path() . "\n";
    $perms->Add("$group", FULL|FULL, ACCESS_ALLOWED_ACE_TYPE, OBJECT_I
+NHERIT_ACE | CONTAINER_INHERIT_ACE);
    if ($perms->Set()) {
        print "Account: '$group' was granted Full ACE permission to: '
+". $perms->Path() . "'",
        0x00000000|0x00000040|0x00000000,"' Operation Successful";
    } else {
        print "This particular operation has failed\n\n***PLEASE CONTA
+CT THE ADMINISTRATOR***\n",
        0x00000000|0x00000040|0x00000000,"Operation Failure";
    }
    $perms->Close();
}

AddGroupToDir( 'ALL', 'D:/TEMP' );
__END__
C:\test>189474

Account 'ALL'    Path D:\TEMP
Account: 'ALL' was granted Full ACE permission to: 'D:\TEMP'64' Operat
+ion Successful
C:\test>
[download]

I ran it whilst logged on as Administrator.

My first guess is that the account you are running it under doesn't have permission to set or modify the permissions on the object you are modifying?