必ず10個返す

fill.pl

use strict;
use warnings;
use List::Util;
use Test::More;

sub fill {
    my ($list) = @_;
    my $max = 10;

    [@$list, map { undef } 1..($max - scalar(@$list))]
}

is scalar(@{fill([1..10])}), 10;
is scalar(@{fill([1..3])}), 10;
is scalar(@{fill([])}), 10;

done_testing;