$ cat file1.txt 
1
2
3
4
5

$ cat file2.txt 
5
4
3
2
1


##</code><code>##

#!/usr/bin/perl -w 
use strict;
use Tie::File;

my ($file1,$file2,$fileout)  = @ARGV;

tie my @ry1,"Tie::File",$file1 or die "$file1:$!";
tie my @ry2,"Tie::File",$file2 or die "$file2:$!";
tie my @out,"Tie::File",$fileout or die "$fileout:$!";

@out=(@ry1,@ry2);

untie @out;
untie @ry2;
untie @ry1;


##</code><code>##

$ cat out.txt 
1
2
3
4
5
5
4
3
2
1