what does the double forward slash mean here?

From this page here: http://perldoc.perl.org/perlop.html#Logical-Defined-Or

Although it has no direct equivalent in C, Perl’s // operator is related to its C-style or. In fact, it’s exactly the same as ||, except that it tests the left hand side’s definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it’s defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2 cannot, and EXPR1 will only be evaluated once). This is very useful for providing default values for variables. If you actually want to test if at least one of $a and $b is defined, use defined($a // $b).