#!/usr/bin/perl
use warnings;
use strict;

my $string = join q(), map int rand 10, 1 .. 400_000;
my $mask   = join q(), map int rand 2,  1 .. 400_000;
$mask =~ tr/01/\x00\xff/;

my $result = $string & $mask;
$result =~ tr/\x00//d;
print $result;