use Benchmark;

    timethese(500_000, {
            one => sub {
        my $foo = "    perl monks    ";
            $foo =~ s/^\s+|\s+$//g;
        },

        two => sub {
            my $foo = "    perl monks    ";
            for ($foo) {
                s/^\s+//;
                s/\s+$//;
            }
        },
    });