HTTP POST (Perl & LWP)

2.6. HTTP POST

Some forms use GET to submit their parameters to the server, but many use POST. The difference is POST requests pass the parameters in the body of the request, whereas GET requests encode the parameters into the URL being requested.

Babelfish (http://babelfish.altavista.com) is a service that lets you translate text from one human language into another. If you're accessing Babelfish from a browser, you see an HTML form where you paste in the text you want translated, specify the language you want it translated from and to, and hit Translate. After a few seconds, a new page appears, with your translation.

Behind the scenes, the browser takes the key/value pairs in the form:

urltext = I like pie
lp = en_fr
enc = utf8

and rolls them into a HTTP request:

POST /translate.dyn HTTP/1.1
Host: babelfish.altavista.com
User-Agent: SuperDuperBrowser/14.6
Content-Type: application/x-www-form-urlencoded
Content-Length: 40
  
urltext=I%20like%20pie&lp=en_fr&enc=utf8

Just as we used a do_GET( ) function to automate a GET query, Example 2-7 uses a do_POST( ) function to automate POST queries.

Example 2-7. The do_POST subroutine

use LWP;
my $browser;
sub do_POST {
  # Parameters:
  #  the URL,
  #  an arrayref or hashref for the key/value pairs,
  #  and then, optionally, any header lines: (key,value, key,value)
  $browser = LWP::UserAgent->new( ) unless $browser;
  my $resp = $browser->post(@_);
  return ($resp->content, $resp->status_line, $resp->is_success, $resp)
    if wantarray;
  return unless $resp->is_success;
  return $resp->content;
}

Use do_POST( ) like this:

doc = do_POST(URL, [form_ref, [headers_ref]]);
(doc, status, success, resp) = do_GET(URL, [form_ref, [headers_ref]]);

The return values in scalar and list context are as for do_GET( ). The form_ref parameter is a reference to a hash containing the form parameters. The headers_ref parameter is a reference to a hash containing headers you want sent in the request.