[PERL] What is this thing? [Beginner]

I am currently learning to program (Perl) and I've come across this:

$variable ||= 'stuff';

What does ||= mean?

>>17
When you say a number is prime, you must specify the ring of integers you are working in.  For instance, 11 is not prime in Z[(1+√5)/2], the ring of integers of Q(√5), since 11=(4+√5)(4-√5).