This actually is easily googleable... if you're deep enough into the Perl culture. Try searching for it under the name "orcish maneuver
", which will explain not only what the operator is but also exactly how it's being used in this case. (Link is to duckduckgo's first result, which just happens to also be on PerlMonks.)
Also note that, because of the distinction between truthiness and definedness mentioned in previous replies, you're going to want to use //= instead of ||= in the vast majority of cases unless your code needs to support pre-5.10 versions of Perl.