sub read_file_line {
  my $fh = shift;

  if ($fh and my $line = <$fh>) {
    chomp $line;
    return [ split(/\t/, $line) ];
  }
  return;
}

#sub compute {
   # do something with the 2 values
#}

open(my $f1, $input);
open(my $f2, $input_1);

my $pair1 = read_file_line($f1);
my $pair2 = read_file_line($f2);
my $value;

open OUT,">$file";

while ($pair1 and $pair2) {
    $value=$pair1->[3]*(1-$pair2->[3]);
    $pair1 = read_file_line($f1);
    $pair2 = read_file_line($f2);
    print OUT $pair1->[0]."\t".$pair1->[1]."\t".$pair1->[2]."\t".$value."\n";
}

close($f1);
close($f2);
close OUT;