#!/usr/bin/perl

# http://perlmonks.org/?node_id=1198243

use strict;
use warnings;

open my $one, '<', \<<END or die; # replace with normal open
Item              Availability       Broken
Laptop_Monitor        10                2
Laptop_Case           20                0
Laptop_Bag            25                1
Office_Chair          20                1
Mouse                 10                0
Keyboard              10                0
END

open my $two, '<', \<<END or die; # replace with normal open
Item               Newly_order      Ref_Name
Laptop_Monitor        15          HMX-190B-F
Laptop_Bag            20          BMT-1920-U
Office_Chair          10          ORD-455_T
END

$_ = join '', <$one>, <$two>;

print "Item            Availability   Broken    Newly_order     Total_Stock\n";

printf "%-16s%7d%12d%11d%22s\n", $1, $2, $3, $4, "($2-$3+$4)"
  while /^(\S+)\s+(\d+)\s+(\d+)(?=.*^\1\s+(\d+))/gms;