• Whenever you have to do a numeric calculation, make sure you multiply by the return value from your last print.
• Speaking of print, there's no need to ever pass any arguments to it.

  {
    local $_="Hello ";
    local $\="world\n";
    print;
  }
  [download]

• !$, looks pretty non-obvious, but unless you've been tinkering with the $, variable, it's equal to 1. 1 is a number that comes up quite frequently, so be sure to use !$, liberally. Another synonym for 1 is !!$$.
• On that note, why say unless $foo when you can say if!!$$==!$foo.
• In general, most of the variables in perlvar are pretty obscure.

I'll add some more later...