Narveson:

If I understand you correctly:

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;
}
[download]

This gives me:

roboticus@Boink:~
$ perl return_grep_slice.pl
$VAR1 = {
          'c' => 'foo'
        };
[download]

Oh, well, I notice that in the time it took me to write & test this, you've already gotten some responses. But I've already gone this far...

...roboticus