#! /usr/bin/env perl

use strict;
use warnings;
use autodie;
use 5.10.0;

open my $fa, '<', 'file-a';
open my $fb, '<', 'file-b';

my %fa;
my %fb;

while (<$fb>) {
  chomp;
  my @F = split ',';

  push @{ $fb{"$F[2]:$F[1]"} }, $_;
}

while (<$fa>) {
  chomp;
  my @F = split ',';

  push @{ $fa{"$F[0]:$F[1]"} }, $_;
}

foreach my $key (keys %fa) {
  for my $i ( 0 .. $#{ $fa{$key} } ) {

    my @A = split ',', $fa{$key}[$i];
    my @B = split ',', $fb{$key}[$i];

    say join ',',
      $B[0], $B[1], $B[2],
      $A[2], $A[3], $A[4],
      $i + 1;
  }

  if ( @{ $fa{$key} } + 1 == @{ $fb{$key} } ) {
    my @A = split ',', $fa{$key}[-1];
    my @B = split ',', $fb{$key}[-1];

    say join ',',
      $B[0], $B[1], $B[2],
      $A[2], $A[3], $A[4],
      scalar @{ $fb{$key} };
  }
}

##</code><code>##

Unit4874,29031990,02,feb,21,5,1
Unit4875,29031990,02,feb,18,5,2
Unit4876,29031990,02,feb,20,5,3
Unit4877,29031990,02,feb,21,5,4
Unit4878,29031990,02,feb,21,5,5