use strict;
use warnings;
use threads;

my $str = 'x' x 32_000_000;

sub fork_wait {
    my $pid = fork;
    if ($pid) {
        wait;
    }
    else {
        substr( $str, $_, 1 ) &= ~ (' 'x1000) for 0 .. (length( $str )/1000) -1;
        exit 0;
    }
}

sub create_thread_join {
    threads->create( sub {
        substr( $str, $_, 1 ) &= ~ (' 'x1000) for 0 .. (length( $str )/1000) -1;
    } )->join;
}

use Benchmark qw( cmpthese );

cmpthese - 3,
  {
    Forks   => \&fork_wait,
    Threads => \&create_thread_join,
  };