use strict;
use Tie::File;

tie my @f1, 'Tie::File', 'file1.txt' or die;
tie my @f2, 'Tie::File', 'file2.txt' or die;
tie my @f3, 'Tie::File', 'file3.txt' or die;

@f1 = grep { my $word = $_;
             my $size = $#f2;
             @f2 = grep { if ($_ eq $word) { push @f3, $_; 0 }
                          else { 1 } } @f2;
             $size == $#f2; } @f1;

##</code><code>##

three
four
five
six
seven

##</code><code>##

nine
eleven
twenty
thirty
forty

##</code><code>##

one
two