Talking about the primes...
I have to talk about Abigail's oneliner:

perl -wle 'print "Prime" if (1 x shift) !~ /^1?$|^(11+?)\1+$/' [num]
[download]

More details on http://montreal.pm.org/tech/neil_kandalgaonkar.shtml
(from google)

Hopes perl -le '$_=$,=q,\,@4O,,s,^$,$\,,s,s,^,b9,s,$_^=q,$\^-]!,,print'