use File::Slurp;


my $outputfile1 = "file1.txt"
my $outputfile2 = "file2.txt"
my $outputfile3 = "file3.txt"

open (my $combined_file,">$outputfile3") or die "cannot create combined output file - $!";
        my @a = read_file($outputfile1) or die "couldn't read $outputfile1 - $! [$^E]";
	my @b = read_file($outputfile2) or die "couldn't read $outputfile2 - $! [$^E]";
	my $combined = {}; # hashref

	my $i=0;
	foreach (@a) 
	{
		chomp $_;
		$combined->{$i}{b} = '' unless defined $combined->{$i}{b};
		$combined->{$i++}{a} = $_;
	}

	$i=0;
	foreach (@b) 
	{
		chomp $_;
		$combined->{$i}{a} = '' unless defined $combined->{$i}{a};
		$combined->{$i++}{b} = $_;
	}

	foreach my $i (sort {$a<=>$b} keys %$combined) 
	{
		print $combined_file $combined->{$i}{a}, ("\n"),$combined->{$i}{b};
	}
}
close($combined_file);