#assumes %the_hash{'this_id'} = { key1=>'quux', key4=>12 } and so on
my @selected_id_keys = grep {
    ( exists $the_hash{$_}{'key1'}
         and $the_hash{$_}{'key1'} eq 'foo')
    and ( exists $the_hash{$_}{'key2'}
         and $the_hash{$_}{'key2'} eq 'bar')
    or ( exists $the_hash{$_}{'key4'}
         and $the_hash{$_}{'key4'} < 10)
} keys %the_hash;