#!/usr/local/bin/perl

use strict;
use warnings;

my %hash = (
    'a' => {
        'x' => 40,
        'z' => 102
    },
    'b' => {
        'z' => 100,
        'x' => 10,
        'y' => 20
    },
    'c' => {
        'x' => 50
    },
    'd' => {
        'z' => 101,
        'y' => 30
    }
);

my $x_but_not_y = 0;
my $z = 0;

foreach my $key (keys %hash) {
    $x_but_not_y++ if defined $hash{$key}->{x} and not defined $hash{$key}->{y};
    $z++ if defined $hash{$key}->{z};
}

print "number of keys where x defined, but not y: $x_but_not_y\n";
print "number of keys where z defined: $z\n";