Here's another way to do it using Tie::File and List::MoreUtils::pairwise().

Here's the input data:

$ cat pm_1075362_1.txt
file 1 line 1
file 1 line 2

$ cat pm_1075362_2.txt
file 2 line 1
file 2 line 2
file 2 line 3

$ cat pm_1075362_3.txt
file 3 line 1
file 3 line 2
[download]

Here's the script showing examples for: a) first file shorter than the second; b) first file longer than the second; c) both files with same length.

#!/usr/bin/env perl -l

use strict;
use warnings;
use autodie;

use Tie::File;
use List::MoreUtils qw{pairwise};

my ($file1, $file2, $file3) = map { "pm_1075362_$_.txt" } 1 .. 3;

tie my @file1, 'Tie::File', $file1;
tie my @file2, 'Tie::File', $file2;
tie my @file3, 'Tie::File', $file3;

print '*** File 1 (2 lines) and File 2 (3 lines):';
print_tabbed_pairs(\@file1, \@file2);
print '*** File 2 (3 lines) and File 1 (2 lines):';
print_tabbed_pairs(\@file2, \@file1);
print '*** File 1 (2 lines) and File 3 (2 lines):';
print_tabbed_pairs(\@file1, \@file3);

sub print_tabbed_pairs {
    my ($file1, $file2) = @_;
    print for pairwise {
        join "\t" => (defined $a ? $a : ''), (defined $b ? $b : '')
    } @$file1, @$file2;
}
[download]

Output:

*** File 1 (2 lines) and File 2 (3 lines):
file 1 line 1    file 2 line 1
file 1 line 2    file 2 line 2
    file 2 line 3
*** File 2 (3 lines) and File 1 (2 lines):
file 2 line 1    file 1 line 1
file 2 line 2    file 1 line 2
file 2 line 3    
*** File 1 (2 lines) and File 3 (2 lines):
file 1 line 1    file 3 line 1
file 1 line 2    file 3 line 2
[download]

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