=item $h->if_unmodified_since

These header fields are used to make a request conditional.  If the requested
resource has (or has not) been modified since the time specified in this field,
then the server will return a C<304 Not Modified> response instead of
the document itself.

=item $h->last_modified

This header indicates the date and time at which the resource was last
modified. I<E.g.>:

  # check if document is more than 1 hour old
  if (my $last_mod = $h->last_modified) {
      if ($last_mod < time - 60*60) {
	  ...
      }
  }

=item $h->content_type

The Content-Type header field indicates the media type of the message
content. I<E.g.>:

  $h->content_type('text/html');

The value returned will be converted to lower case, and potential
parameters will be chopped off and returned as a separate value if in
an array context.  If there is no such header field, then the empty
string is returned.  This makes it safe to do the following:

  if ($h->content_type eq 'text/html') {
     # we enter this place even if the real header value happens to
     # be 'TEXT/HTML; version=3.0'
     ...
  }

=item $h->content_is_html

Returns TRUE if the Content-Type header field indicate that the
content is some kind of HTML (including XHTML).  This method can't be
used to set Content-Type.

=item $h->content_is_xhtml

Returns TRUE if the Content-Type header field indicate that the
content is XHTML.  This method can't be used to set Content-Type.

=item $h->content_is_xml

Returns TRUE if the Content-Type header field indicate that the
content is XML.  This method can't be used to set Content-Type.

=item $h->content_encoding

The Content-Encoding header field is used as a modifier to the
media type.  When present, its value indicates what additional
encoding mechanism has been applied to the resource.

=item $h->content_length

A decimal number indicating the size in bytes of the message content.

=item $h->content_language

The natural language(s) of the intended audience for the message
content.  The value is one or more language tags as defined by RFC
1766.  Eg. "no" for some kind of Norwegian and "en-US" for English the
way it is written in the US.

=item $h->title

The title of the document.  In libwww-perl this header will be
initialized automatically from the E<lt>TITLE>...E<lt>/TITLE> element
of HTML documents.  I<This header is no longer part of the HTTP
standard.>

=item $h->user_agent

This header field is used in request messages and contains information
about the user agent originating the request.  I<E.g.>:

  $h->user_agent('Mozilla/1.2');

=item $h->server

The server header field contains information about the software being
used by the originating server program handling the request.

=item $h->from

This header should contain an Internet e-mail address for the human
user who controls the requesting user agent.  The address should be
machine-usable, as defined by RFC822.  E.g.:

  $h->from('King Kong <king@kong.com>');

I<This header is no longer part of the HTTP standard.>

=item $h->referer