Probably not exactly what you're looking for, but, check lwpcook's section on large documents. This will allow you to measure your throughput in an interactive manner.

Here's a variation of the code give in the above link

use LWP::UserAgent;
use strict;

my $url      = 'http://www.w3.org/TR/xhtml1/';
my $agent    = LWP::UserAgent->new;
my $received = 0;
my $start    = time;
my $res      = $agent->request(
    HTTP::Request->new( GET => $url ),
    sub {
        my( $chunk, $res ) = @_;

        $received += length( $chunk );

        printf(
            "%d%% - %d Kbps\n",
            100 * $received / $res->content_length,
            $received / ( time - $start ) / 1024
        );
    }
);

print $res->message;
[download]

HTH