What is in your CPN and ITEM subroutines? You have to define them to check for criteria you want. They don't just magically appear out of nowhere. Here is a fully-functional example, hopefully it will make something click for you:

my %hash = (
    first  => { a => 2, b => 3 },
    second => { a => 3, b => 2 },
    third  => { a => 5, b => 4 },
    fourth => { a => 4, b => 6 },
);

my %newhash = map  { $_ => $hash{$_} }
              grep {     foo($hash{$_})
                     and bar($hash{$_}) }
              keys %hash;

print "$_\n" foreach keys %newhash;

sub foo {
    my ($href) = @_;
    $href->{a} > 3;
}

sub bar {
    my ($href) = @_;
    $href->{b} > 2;
}
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As you can see, in this example foo makes sure the value at a is greater than 3, and bar makes sure the value at b is greater than 2. The end result should print third and fourth, since those are the only keys that pass the criteria.