Here's one non-scientific way to check:

#!/usr/bin/perl

use warnings;
use strict;

use Test::More;

my $file = 'file.txt';

my $perms = sprintf("%o", (stat $file)[2] & 0777); 

my ($u, $g, $w) = split //, $perms;

my @writable = qw(2 3 6 7);

ok (!(grep /$g/, @writable), "File is not group writable");

ok (!(grep /$w/, @writable), "File is not world writable");
[download]

-stevieb