This takes 5.25 seconds on my machine (4GB, 4 cores, Linux).

The respective threads versions take:

use 5.010;
use threads;
use Time::HiRes qw(time);

my $data = 'x' x (500 * 1024**2);

my $t    = time;
for my $n ( 1 .. 100 ) {
    threads->create(
    sub {
        substr( $data, 4096 * $_ + $n, 1 ) |= 1 for 0 .. 124;
    }
    )->join;
}
say time - $t;

__END__
164.842848062515
[download]

use 5.010;
use threads;
use Time::HiRes qw(time);

my $t    = time;
for my $n ( 1 .. 100 ) {
    threads->create(
    sub {
        my $data = 'x' x (500 * 1024**2);
        substr( $data, 4096 * $_ + $n, 1 ) |= 1 for 0 .. 124;
    }
    )->join;
}
say time - $t;

__END__
172.88907790184
[download]

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