You can't pass parameters to a callback, so you have to either pass a closure or do some other magic. I haven't tried anything like this recently, and thus I write simply untested code from the top of my head:

use strict;
use LWP::UserAgent;

my $url = 'http://www.example.com/';
my $filename = 'test.file';

open CONTENT, "<", $filename
  or die "Couldn't open $filename : $!";
binmode CONTENT;

my $callback = sub {
  my $content;
  my $size = read( CONTENT, $content, 51200 );
  $content = ""
    unless $size;
  $content;
};

my $ua = LWP::UserAgent->new();
$ua->post($url,$callback);
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For anything fancier, I strongly suggest you learn about anonymous code references and closures.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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