I did it like this, using Net::FTP and without having to modify it:

use Net::FTP;
use IO::Callback;

my $BytesPerHash = 2048;
my $HashHandle = IO::Callback->new('>',
        sub {
                # activity code
                # called every $BytesPerHash
        }
);

my $ftp = Net::FTP->new($host, Timeout => $Timeout);
$ftp->hash($HashHandle, $BytesPerHash);
$ftp->login($Username, $Password);
$ftp->binary;
$ftp->put($localFile, $remoteFile);
[...]
[download]

Works nicely