se strict;
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Produces the error message:

Can't locate object method "se" via package "strict" (perhaps you forg
+ot to load "strict"?)
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my $data = `/sbin/ifconfig | grep “RX bytes” | head -1`;
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I guess you are assuming that the "certain device" you want information on is the first device listed by ifconfig?

$data =~ /RX bytes\:(\d+) .*TX bytes\:(\d+) .*/;
my $recv = $1/1024;
my $sent=$2/1024;
$recv =~ s/\..*//;
$sent =~ s/\..*//;
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You shouldn't use the variables $1 and $2 unless the regular expression matched correctly otherwise the values in $1 and $2 will be from the last successful match. Is there any reason that you didn't use the int function instead of the substitution operator?

You probably want something like this:

sub get_measures {
    my $device = qr/\A\Q$_[0]\E\s/;  # pass device name as first argum
+ent

    local $/ = '';  # paragraph mode
    my ( $recv, $sent ) = map {
        /$device/ && /RX bytes\:(\d+) .*?TX bytes\:(\d+)/
            ? int( $1 / 1024 ), int( $2 / 1024 )
            : ()
        } qx[/sbin/ifconfig];

    return $recv, $sent;
    }
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I like it. Perhaps on Windows, ifconfig could be replaced with netstat -e, though I don't know what that would look like if there were multiple interfaces or how difficult it would be to parse. i'm sure with Perl it wouldn't be THAT difficult.

I wrote something similar to this once. Here's the quick one-liner from my bash_history. I'm not sure if it measures the incoming or outgoing traffic on the eth0 device, because it was just a quick one-liner which I don't usually document or even save. It's using the linux proc interface instead of executing ifconfig.

ruby -we '@s = File.open("/proc/net/dev"); @pq = (); while sleep 20; @
+s.rewind; cq = @s.readlines.find{|l|l=~/^\s*eth0/}.scan(/[^\W:]+/)[9]
+.to_i; ct = Time.now; @pq and printf "%.2f byte/sec\n", (cq-@pq)/Floa
+t(ct-@pt); @pq, @pt = cq, ct; end;'
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By the way, you could also get more detailed statistics by setting up iptables rules because there's a packet and byte counter for each rule.

very nice and small

but does it do graphs with the rates in them ? :P

That's easy too. Firstly change the print statement so it would print the time (in epoch seconds) and the rate separated by spaces. Write that output to a file, and then graph that file with gnuplot.

I actually have a perl program that draws a graph by calling gnuplot, here are a few parts of its code (I did it as a work so I don't want to publish all of it). The part not shown here opens a tempfile and writes into it the numbers in the simple format mentioned above. I also print some overall statistics to stdout. The following sub graphs the data then (it might need some adaptation for your purposes).

use File::Temp ();

sub showgraph {
        my($cmdfile, $cmdfilename);
        my $cmds = qq[set key off\nplot "] . quote($outname) . qq[" wi
+th lines\n! echo "Press return to continue"
; read\n];
        ($cmdfile, $cmdfilename) = File::Temp::tempfile undef, "UNLINK
+", 1;
        $out or die "error creating temporary commands file";
        print "Writing gnuplot command to temporary file $cmdfilename\
+n";
        print $cmdfile $cmds or
                die "error writing command file: $!";
        flush $cmdfile or die
                "error flushing comand file";
        print "Launching gnuplot\n";
        system "gnuplot", $cmdfilename;
        close $cmdfile;
}
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Update 2008-01-29: see Plot a spiral with gnuplot.