As already pointed out, it's not exactly clear what you mean, but maybe try something like this:

#!/usr/bin/perl

use GTop ();

my $gtop = GTop->new();

my %pre_stats;

sub get_netstats {
    my $init = shift;
    
    my $netload = $gtop->netload("eth0");
    
    for (qw(bytes_out bytes_in bytes_total
            packets_out packets_in packets_total)) {
        if ($init) {
           $pre_stats{$_} = $netload->$_();
        } else {
            printf "%-14s: %d\n", $_, $netload->$_() - $pre_stats{$_};
        }
    }
}

get_netstats("initial");
system "wget", "http://perlmonks.org";   # your app here
get_netstats();


__END__
$ ./834243.pl
--18:20:17--  http://perlmonks.org/
           => `index.html'
Resolving perlmonks.org... 66.39.54.27, 209.197.123.153
Connecting to perlmonks.org|66.39.54.27|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 83,769 (82K) [text/html]

100%[=================================================================
+==========>] 83,769       118.72K/s             

18:20:18 (118.47 KB/s) - `index.html' saved [83769/83769]

bytes_out     : 3553
bytes_in      : 88978
bytes_total   : 92531
packets_out   : 51
packets_in    : 68
packets_total : 119
[download]

Of course, as the traffic info is interface-global (not process-specific), there mustn't be any other significant network traffic going on while you're measuring...