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