#! /usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

# Data Structure
my %h = (
    'a' => 'z',
    'b' => 'y',
    'c' => 'x',
    'd' => 'w',
);

my @to_keep = qw{ a c };

my %keepers = map { $_ => 1 } @to_keep;

foreach ( keys %h ) {
    if ( !exists $keepers{$_} ) {
        delete $h{$_};
    }
}

$Data::Dumper::Varname = 'h';
print Dumper( \%h );

$h1 = {
          'c' => 'x',
          'a' => 'z'
      };

##</code><code>##

my @to_keep = qw{ a c };
my %keepers;
foreach ( @to_keep ) {
    $keepers{$_}++;
}

$Data::Dumper::Varname = 'keepers';
print Dumper( \%keepers );

keepers1 = {
          'c' => 1,
          'a' => 1
        };

##</code><code>##

my @to_keep = qw{ a c };
my %keepers;
%keepers = map { $_++ } @to_keep;

$Data::Dumper::Varname = 'keepers';
print Dumper( \%keepers );

$keepers1 = {
          'a' => 'c'
        };