There is more than one way to do it.

You can compute calculate the numbers from a simple sequence:

my @f;
for my $i (0 .. 20) {
    my @e = (2 * $i, 2 * $i + 1);
    push @f, \@e;
}
print map "(@$_)", @f
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or you can use the C-style loop to skip over the unwanted numbers:

for (my $i = 0; $i <= 40; $i += 2) {
    my @e = ($i, $i + 1);
    push @f, \@e;
}
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or you can use grep to filter the numbers you want:

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

etc.