#!/usr/bin/perl -w

use Data::Dumper;

{
  my $hashref;
  $hashref->{'foo'} = "bar";

  print Dumper($hashref);
}

##</code><code>##

[alex@home dev]$ perl -w hashref.pl 
$VAR1 = {
          'foo' => 'bar'
        };

##</code><code>##

#!/usr/bin/perl -w

use Data::Dumper;

{
  my $hashref->{'foo'} = "bar";

  print Dumper($hashref);
}

##</code><code>##

[alex@home dev]$ perl -w hashref2.pl
Can't use an undefined value as a HASH reference at hashref2.pl line 6.