#!/usr/bin/perl

# https://perlmonks.org/?node_id=1231669

use strict;
use warnings;

$_ = do { local $/; local @ARGV = qw(File1 File2); <> . <> };

print "$1 => Prev: $2 Now: $3\n"
  while /^(\w+), (\w+),.*\n(?=(?:.*\n)*\1, (?!\2,)(\w+).)/gm;