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my @even   = 0, 2, 4 ... { $_ + 2};
my @powers = 1, 2, 4 ... { $_ * 2 };
my @fib    = 1, 1, 2 ... { $^a + $^b};

my @ints   = 1, 2, 3, 4 ... *;
my @even   = 0, 2, 4, 6 ... *;
my @powers = 1, 2, 4, 8 ... *;

1, 3, 5 ... *        # odd numbers
1, 2, 4 ... *        # powers of 2

sub series(@items, $recursion_level = 2) {
    return if $recursion_level < 0;
...
    # give up
    return;
}

use List::MoreUtils qw(all);
sub series {
...
    return;
}

#!/usr/bin/perl
use strict;
...
    $result = 'undef' unless defined $result;
    is $result, $expected, "seq " . join(', ', @{$t->[0]});
}