So my question is, what the heck does //= do?
This is the relatively new "defined or" operator
$a //= $b;
means that $a is unchanged if it is defined, and is assigned to $b if not defined.
At least, that's my understanding, I have never used it, but the following test under the Perl debugger seems to confirm it:
$ perl -de 42
Loading DB routines from perl5db.pl version 1.33
Editor support available.
Enter h or `h h' for help, or `man perldebug' for more help.
DB<1> $a //=5;
DB<2> p $a
DB<3> $a //=6;
DB<4> p $a