#!/usr/bin/env perl

use strict;
use warnings;

use Inline::Files;

my @file_data
    = map { [ do { local $/; <$_> } =~ /(\w+)/gm ] } *K1_TXT, *K2_TXT, *K3_TXT;

my $glob_string = join '' => map { '{' . join(',' => @$_) . '}' } @file_data;

print "$_\n" for glob $glob_string;
 
__K1_TXT__
A1T1
A2T3
__K2_TXT__
C1G1
C1G2
C2G1
__K3_TXT__
A1C1