use strict;
use warnings;
use Data::Dumper;

my @array = qw(a b c e 1 2 3 4 5 3 f);
my $len = 3;
my @stack = map{$_%$len?():[grep{$_}@array[$_..$_+$len-1]]} 0..@array-1;
print Dumper \@stack;