#!/usr/bin/perl -l

use strict;
use warnings;

my $k = 5;

my $file1contents = 'TACATCTCAAAACACTTTCATCTCACGACTACTACTACTACTTCAAAACACCATCAT';
my $file2contents = 'ACTTCAACATAACTACTATATACTACTCATACTACTACTCTTAAAACTACTATACTA';

$_ = "$file1contents\n$file2contents";

print "at position $-[0] is match $1" while /(.{$k,}) (?= .* \n .* \1 )/gx;