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

my %config = (a=>undef, b=>0, c=>'foo');
my @wanted = qw(a b c d);

my %result = get_small_hash(\%config);

sub get_small_hash {
    my $hr = shift;
    return map { $_ => $$hr{$_} }
           grep { exists($$hr{$_}) and defined($$hr{$_}) and $$hr{$_} }
           @wanted;
}