As chromatic said, the simplest way to do this is with LWP. You want HTTP::Request::Common, from which the doc reads:

       POST $url, [$form_ref], [Header => Value,...]
           This works mostly like GET() with POST as the method,
           but this function also takes a second optional array
           or hash reference parameter ($form_ref).  This argu-
           ment can be used to pass key/value pairs for the form
           content.  By default we will initialize a request
           using the `application/x-www-form-urlencoded' content
           type.  This means that you can emulate a HTML <form>
           POSTing like this:

             POST 'http://www.perl.org/survey.cgi',
                  [ name   => 'Gisle Aas',
                    email  => 'gisle@aas.no',
                    gender => 'M',
                    born   => '1964',
                    perc   => '3%',
                  ];

           This will create a HTTP::Request object that looks
           like this:

             POST http://www.perl.org/survey.cgi
             Content-Length: 66
             Content-Type: application/x-www-form-urlencoded

             name=Gisle%20Aas&email=gisle%40aas.no&gender=M&born=1964&
+perc=3%25

           The POST method also supports the `multi-
           part/form-data' content used for Form-based File
           Upload as specified in RFC 1867.  You trigger this
           content format by specifying a content type of
           `'form-data'' as one of the request headers.  If one
           of the values in the $form_ref is an array reference,
           then it is treated as a file part specification with
           the following interpretation:

             [ $file, $filename, Header => Value... ]

           The first value in the array ($file) is the name of a
           file to open.  This file will be read and its content
           placed in the request.  The routine will croak if the
           file can't be opened.  Use an `undef' as $file value
           if you want to specify the content directly.  The
           $filename is the filename to report in the request.
           If this value is undefined, then the basename of the
           $file will be used.  You can specify an empty string
           as $filename if you don't want any filename in the
           request.

           Sending my ~/.profile to the survey used as example
           above can be achieved by this:

             POST 'http://www.perl.org/survey.cgi',
                  Content_Type => 'form-data',
                  Content      => [ name  => 'Gisle Aas',
                                    email => 'gisle@aas.no',
                                    gender => 'M',
                                    born   => '1964',
                                    init   => ["$ENV{HOME}/.profile"],
                                  ]

           This will create a HTTP::Request object that almost
           looks this (the boundary and the content of your
           ~/.profile is likely to be different):

             POST http://www.perl.org/survey.cgi
             Content-Length: 388
             Content-Type: multipart/form-data; boundary="6G+f"

             --6G+f
             Content-Disposition: form-data; name="name"

             Gisle Aas
             --6G+f
             Content-Disposition: form-data; name="email"

             gisle@aas.no
             --6G+f
             Content-Disposition: form-data; name="gender"

             M
             --6G+f
             Content-Disposition: form-data; name="born"

             1964
             --6G+f
             Content-Disposition: form-data; name="init"; filename=".p
+rofile"
             Content-Type: text/plain

             PATH=/local/perl/bin:$PATH
             export PATH

             --6G+f--
[download]

No point in reinventing the wheel, when it's all nicely there for the taking.

-- Randal L. Schwartz, Perl hacker