Metabase::Fact::String3pm

  # defining the fact class
  package MyFact;
  use Metabase::Fact::String;
  our @ISA = qw/Metabase::Fact::String/;
  sub content_metadata {
    my $self = shift;
    return {
      'size' => [ '//num' => length $self->content ],
    };
  }
  sub validate_content {
    my $self = shift;
    $self->SUPER::validate_content;
    die __PACKAGE__ . " content length must be greater than zero\n"
      if length $self->content < 0;
  }

...and then...

  # using the fact class
  my $fact = MyFact->new(
    resource => 'RJBS/Metabase-Fact-0.001.tar.gz',
    content  => "Hello World",
  );
  $client->send_fact($fact);