If i understand you, yuou want to build a similar thing yourself? why not look at the code, you'll see anything you need there.
e.g.: www mechanize uses these modules, to support it's functions:

use HTTP::Request 1.30;
use LWP::UserAgent 2.003;
use HTML::Form 1.00;
use HTML::TokeParser;
use URI::URL;
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update:
I do think it uses libwww-perl for posting, e.g.:

  use HTTP::Request::Common;
  my $ua = LWP::UserAgent->new;
  $ua->request(POST 'http://somewhere/foo', [foo => bar, bar => foo]);
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