my @f;
for my $i (0 .. 20) {
    my @e = (2 * $i, 2 * $i + 1);
    push @f, \@e;
}
print map "(@$_)", @f

##</code><code>##

for (my $i = 0; $i <= 40; $i += 2) {
    my @e = ($i, $i + 1);
    push @f, \@e;
}

##</code><code>##

for my $i (grep 0 == $_ % 2, 0 .. 40) {
    my @e = ($i, $i + 1);
    push @f, \@e;
}