my $c = -$l;
push @b, [ grep {defined $_} @a[($c+=$l)..($c+$l-1)] ] for 0 .. $#a/$l

##</code><code>##

push @b, [ grep {defined $_} @a[$_*$l .. ($_*$l+$l>$#a ? $#a : $_*$l+$l-1)] ]
    for 0 .. $#a/$l;