\my $out declares $out (my $out) and returns a reference to it (\$out). run3 takes this to mean the output the output to be placed in $out. Without the \, you'd be passing the value of $out, which is undefined. run3 takes this to mean the child should inherit the parent's STDOUT.

Update: Doh! I said "With the \" when I meant "Without the \". Fixed. Thenks choroba.