And if every worker fork needs to modify 1 bit on every 4x page of 1 GB of shared data, it's gonna take forever.

Rubbish

use strict;
use warnings;
use 5.010;
use Time::HiRes qw(time);

my $data = 'x' x 500_000_000;
my $t = time;
$data =~ tr/x/y/;
say time - $t;
__END__
0.469923973083496
[download]

use strict;
use warnings;
use 5.010;
use Time::HiRes qw(time);

my $data = 'x' x 500_000_000;
my $t    = time;
unless (fork) {
    $data =~ tr/x/y/;
    exit;
}
wait;
say time - $t;
__END__
0.765344142913818
[download]

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

my $data = 'x' x 500_000_000;
my $t    = time;
threads->create(
    sub {
        $data =~ tr/x/y/;
    }
)->join;
say time - $t;
__END__
1.29563307762146
[download]

That's not quite what I said is it :)

See how you fare with this:

use strict;
use warnings;
use 5.010;
use Time::HiRes qw(time);

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

my $t    = time;
for my $n ( 1 .. 100 ) {
    unless (fork) {
        substr( $data, 4096 * $_ + $n, 1 ) |= 1 for 0 .. 124;
        exit;
    }
}
waitall;

say time - $t;
[download]

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]