Well your question is kind of ambiguous. It sounds like you want the browser to upload automatically, but then you say user::agent, as in LWP::UserAgent?

The browser can't do it.

Here is how to use LWP::UserAgent. There are better scripts than this, which will also show upload progress, search for them.

#!/usr/bin/perl
use warnings;
use strict;
use HTTP::Request::Common qw(POST);
use LWP::UserAgent;

my $url= 'http://somesite.net/cgi-bin/uploader.cgi';
my $file     = 'testout.tgz';
my $filezzzz = 'testzzzz.tgz';
my $ua       = new LWP::UserAgent;

my $req = POST $url,
  Content_Type => 'multipart/form-data',
  Content      => [
    p_upload => [ $file, $filezzzz ]    #actual file, name to use as u
+pload 
  ];

my $res = $ua->request($req);
if ( $res->is_success ) {
    print $res->as_string;
}
else {
    print $res->status_line;
}
exit 0;
[download]