sub filter {
  my $where = shift;
  my @in = @_;
  
  # This class method is used to filter an array of hashrefs against a set of criteria defined in $where.
  # Example:
  #   @matching_hosts = filter( { site => 56, type => 4 }, @all_hosts);
  # In this example, @matching_hosts will only contain those hashrefs that would return TRUE for the following code:
  #   ($_->{'site'} eq '56' && $_->{'type'} eq '4')
  # Note that the "eq" and "&&" are implied; no other operators are supported.

  # The order of the array is not affected.

  my @out = ();

  foreach my $record (@in) {
    my $keep = 1;
    foreach my $field (keys %{$where}) {
      unless ($record->{$field} eq $where->{$field}) {
        $keep = 0;
        last;
      }
      push @out, $record if $keep;
    }
  }

  return @out;
}