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 };

%h = map { $_ => $h{$_} } @to_keep;

print Dumper \%h;