#!/usr/bin/env perl    

use strict;
use warnings;
use 5.010;

use Array::Heap::PriorityQueue::String;

my @file_names = qw( in1 in2 in3 );

my @fh;
my $heap = Array::Heap::PriorityQueue::String->new;

for (0 .. $#file_names) {
  my $fn = $file_names[$_];
  open my $new_fh, '<', $fn or die "Unable to open $fn: $!\n";
  $fh[$_] = $new_fh;

  add_from_fh_id($_);
} 

my ($current_word) = split /\s+/, $heap->peek;
my $total = 0;
while (my $item = $heap->get) {
  my ($word, $count, $fh_id) = split /\s+/, $item;

  if ($word eq $current_word) {
    $total += $count;
  } else {
    say "$current_word\t$total";
    $current_word = $word;
    $total = $count;
  } 

  add_from_fh_id($fh_id);
} 

say "$current_word\t$total";


sub add_from_fh_id {
  my $fh_id = shift;

  my $read_from = $fh[$fh_id];
  my $next_line = <$read_from>;
  if ($next_line) {
    chomp $next_line;
    $heap->add("$next_line $fh_id");
  } 
}

##</code><code>##

bar	70
blah	69
foo	146
life	42
word	81
yada	3