Sure. Here's a script that parses two files and compares the values for each key using List::Compare:

#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;
use List::Compare;
use List::AllUtils qw/uniq/;

sub parse_file($) {
    my ($filename) = @_;

    open my $filehandle, "<", $filename
        or die "Could not open $filename: $!";

    my %parsed = ();

    while(<$filehandle>) {
        chomp;

        my ($key, $values) = split "=", $_, 2;
        my @values         = split ",", $values;

        $parsed{$key} = \@values;
    }

    return \%parsed;
}

my %parsed_file1 = %{ parse_file "File1" };
my %parsed_file2 = %{ parse_file "File2" };

foreach my $key (sort { $a cmp $b } uniq (keys %parsed_file1, keys %pa
+rsed_file2)) {
    my $listcompare     = List::Compare->new($parsed_file1{$key} // []
+, $parsed_file2{$key} // []);

    my @added_values    = $listcompare->get_Ronly;
    my @removed_values  = $listcompare->get_Lonly;
    my @retained_values = $listcompare->get_intersection;

    say $key;

    say "\tValues added   : ", join ", ", @added_values;
    say "\tValues removed : ", join ", ", @removed_values;
    say "\tValues retained: ", join ", ", @retained_values;
}
[download]

Here's my File1:

B23168=AOL,ABC
permitted_mac_attempts=1,DEF
B23167=Carphone Warehouse TalkTalk,23
B23163=Orange,Red,Blue,Green,White,Black
B23970=O2
APPLE=FRITTER
[download]

And my File2:

B23168=AOL,ABC,DEF,EFG
permitted_mac_attempts=1,DEF,34,4
B23167=Carphone Warehouse TalkTalk,23,5,6
B23163=Orange,Red,Blue,Green,Yellow
B23970=O2,56
PONY=EARTH
[download]

Using these, the script outputs:

$ perl 1119397.pl
APPLE
        Values added   :
        Values removed : FRITTER
        Values retained:
B23163
        Values added   : Yellow
        Values removed : Black, White
        Values retained: Blue, Green, Orange, Red
B23167
        Values added   : 5, 6
        Values removed :
        Values retained: 23, Carphone Warehouse TalkTalk
B23168
        Values added   : DEF, EFG
        Values removed :
        Values retained: ABC, AOL
B23970
        Values added   : 56
        Values removed :
        Values retained: O2
PONY
        Values added   : EARTH
        Values removed :
        Values retained:
permitted_mac_attempts
        Values added   : 34, 4
        Values removed :
        Values retained: 1, DEF
$
[download]

Writing to a CSV file is left as an exercise for the reader (I don't know how you want your CSV file to look anyway), but I'd strongly suggest using Tux's excellent Text::CSV module.

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