use Benchmark;
@a = qw(the quick brown fox jumps over the lazy dog);
timethese(1000000, {
    'for' => '$_ = lc for @a',
    'map' => '@a = map lc, @a',
});



$ perl580 /tmp/p
Benchmark: timing 1000000 iterations of for, map...
       for: 20 wallclock secs (19.28 usr +  0.02 sys = 19.30 CPU) @ 51813.47/s (n=1000000)
       map: 47 wallclock secs (47.85 usr +  0.02 sys = 47.87 CPU) @ 20889.91/s (n=1000000)