cmpthese -1, {
    a=>q[ 
        open O, '>', 'log'; 
        for(1..10000){
            print O chr(0)x80;
        }
        close O;
    ],
    b=>q[
        for(1..10000){
            open O, '>>','log';
            print O chr(0)x80;
            close O;
        }
    ] 
};;
            (warning: too few iterations for a reliable count)

     s/iter      b      a
b      1.30     --   -99%
a 8.42e-003 15300%     --