#!/usr/bin/perl 

use strict;
use warnings;
use Data::Dumper;

my $h = {
          'a' => 1,
          'b' => 2,
          'c' => 3,
        };

warn Dumper [ $$h{a}, $$h{x}, $$h{b}, $$h{c} ];
warn Dumper [ keys %$h ];

warn Dumper [ grep {$_} $$h{a}, $$h{x}, $$h{b}, $$h{c} ];
warn Dumper [ keys %$h ];

warn !$$h{y};
warn Dumper [ keys %$h ];