#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

my @in_files = qw{A B C};
my $outfile = 'merge_count.out';
my %data;
my $out_fmt = "%s\t%d\n";

for my $infile (@in_files) {
    open my $fh, '<', $infile;

    while (<$fh>) {
        my ($word, $count) = split;
        $data{$word} += $count;
    }
}

open my $fh, '>', $outfile;

for my $key (sort { $data{$a} <=> $data{$b} } keys %data) {
    printf $fh $out_fmt, $key, $data{$key};
}