#!/usr/bin/perl -w

use strict;
use warnings;

my $file = "output.dat";

opendir(DIR, ".");
my @filelist = readdir(DIR);

foreach (@filelist)
{

  if ($_ =~ /^([\d]{6,}).txt/)
  {

   open(FILE,">>$file");
   open(TXT,"<$_");
   foreach my $line (<TXT>) {print FILE "$line";}
  }
  close(FILE);
  close(TXT);
}