#!/usr/bin/perl 

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

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

warn "x: ",$_  for $$h{x};              # autovivifiaction via alias

warn Dumper [ keys %$h ];



tst($$h{y});                            # no autovivification

warn Dumper [ keys %$h ];

sub tst {
    warn "y: ",$_[0] ;
}