use warnings;
use strict;
no warnings "utf8";

my @a = ( [2,5,10,5,12,6,21,5,10,12,23],
       [5,6,11,10,5,10,6,21,5,1,9],
       [6,5,10,15,21]
    );
my $m = 2;
my $n = 2;

my $big = join "\0", map join('', map chr, @$_), @a;
$big =~ tr/\0\n/\n\0/;

my %uniq;
my @m2 = map [map ord||10, split //],
   grep !$uniq{$_}++,
   $big =~ /(?=(.{$n})(?s:.*?\1){${\($m-1)}})./g;

use Data::Dumper;
print Data::Dumper->new([$_])->Terse(1)->Indent(0)->Dump(), "\n"
    for @m2;