use IO::File;
my %old =
	map { $_->[1] => $_->[0] }
	map { chomp; [ split /,\s*/ ] }
	IO::File->new("<$file_old")->getlines;

use Tie::Handle::CSV;
my %new =
	map { ( $_->{'CN'} => $_ ) }
	map { { map { /(.*)=(.*)/ } split /\.(?=[A-Z]+=)/, $_->[3] } }
	Tie::Handle::CSV->new($file_new,header=>0)->getlines;

# now %old and %new are keyed by PC ID.
# you can use the standard techniques for finding keys that
# are in one but not the other, e.g.

my @new_PCs = grep { not exists $old{$_} } sort keys %new;