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

my %hash = ( a => 1, b => 0, c => undef, e => 1, f => 0, g => undef );

my @interesting_keys = qw( a b c d );

my %small_hash = map {
     ($hash{$_}) ? ($_ => $hash{$_}) : ()
} @interesting_keys;

print Dumper(\%small_hash);

##</code><code>##

my %small_hash =
        map { $_ => $hash{$_} }
        grep { $hash{$_} }
        @interesting_keys;

##</code><code>##

$VAR1 = {
          'a' => 1
        };