$ perl -le'
use warnings;
use strict;

my @c = 1 .. 10_000_000;
print @c . "  @c[0..19]";
my @d;
for my $dd ( @c ) {
    push @d, $dd % 2;
    }
print @d . "  @d[0..19]";
my @e;
for my $ee ( @d ) {
    if ( !$ee ) {
        push @e, $ee;
        }
    }
print @e . "  @e[0..19]";
'
10000000  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
10000000  1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
50000000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

##</code><code>##

$ perl -le'
use warnings;
use strict;

my @c = 1 .. 10_000_000;
print @c . "  @c[0..19]";
my @d = map $_%2, @c;
print @d . "  @d[0..19]";
my @e = grep /0/, @d;
print @e . "  @e[0..19]";
'
10000000  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
10000000  1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
5000000  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

##</code><code>##

$ perl -le'
my @c = 1 .. 10_000_000;
print @c . "  @c[0..19]";
my @e = ( 0 ) x ( @c / 2 );
print @e . "  @e[0..19]";
'
10000000  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
5000000  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0