#!/usr/bin/perl

use Data::Dumper;

my %h;
{
   local $h{key} = 'blah';
   # even "local $h{key};" will do
   # inititialization is only for clarity
}

print Dumper \%h;

##</code><code>##

$VAR1 = {
          'key' => undef
        };

##</code><code>##

$VAR1 = {};

##</code><code>##

# ...
my %h;
# ...
my @local_keys = qw/foo bar baz/;
my @inexistent = grep {! exists $h{$_}} @local_keys;
{
   local @h{@local_keys};
   # ...
}
delete @h{@inexistent};
# ...