use strict;
use warnings;
use autodie;
open my $IF1, '<', 'File.in1';
open my $IF2, '<', 'File.in2';
open my $OF,  '>', 'File.out';
while (1) {
   my @Rec1 = split /\s+/, <$IF1>;
   my @Rec2 = split /\s+/, <$IF2>;
   $Rec1[-1] += $Rec2[-1];
   print $OF join("\t", @Rec1), "\n";
   last if eof($IF1) and eof($IF2);
}