use warnings;
use strict;
use Tie::File;

tie (my @file1, 'Tie::File', $filename1) or die $!;
tie (my @file2, 'Tie::File', $filename2) or die $!;

# sortieren
@file1 = sort(@file1);
@file2 = sort(@file2);

open (OUT, ">$outfile") or die $!;

my $i = 0; my $j = 0; # indices fuer file1, file2
while ($file1[$i] or $file2[$j]) {

  # absicherung fuers ende
  unless (defined $file1[$i]) {
    print OUT $file1[$j]; $j++; next;
  } # unless
  unless (defined $file2[$j]) {
    print OUT $file2[$i++]; next;
  } # unless

  # vergleiche zwei Zeilen
  my $compare = $file1[$i] cmp $file2[$j];

  if ($compare < 0) { # file1 ist kleiner
    print OUT $file1[$i++];
  }
  elsif ($compare > 0) { # file2 ist kleiner
    print OUT $file2[$j++];
  }
  else { # gleich gross
    $i++; $j++;
  }

} # while

close (OUT);
untie(@file1);
untie(@file2);