This should get you started. It doesn't give the results you want althought it seems to follow what you said you wanted. Must have misunderstood :-) I just read of the DATA filehandle so you will have to open your input and output files. We localise the change to $, to this block to save suprises. The last line data is in @last which we initialise to '' values to keep warnings happy

use strict;
use warnings;
{
   local $, = ',';      # print will print CSV with this set to ','
   my @last = (0) x 7;  # initialize array to 0's
   while(<DATA>) {
      chomp;
      next if /^\s*$/;           # skip blank lines
      my @data = split',';       # split out line into an array
      my $deltatime = $data[2]-$last[2];
      next unless $deltatime;    # protect against illegal div zero
      my $bps = ( $data[5] + $data[6] ) / $deltatime;
      print @data[0,1,3,4], $bps, "\n";  # this will be CSV
      @last = @data;             # remember last line content
   }
}
# Collected data csv: 
# date,time,epochSec,device,ifIndex,ifInOctets,ifOutOctets,
__DATA__
2001-08-10,18:15:00,997485300,boston,141,1916238333,2743654663,
2001-08-10,18:18:12,997485492,boston,141,1917535505,2744657633,
2001-08-10,18:21:23,997485683,boston,141,1919320447,2747461877,
2001-08-10,18:24:34,997485874,boston,141,1921869420,2748722088,
2001-08-10,18:27:46,997486066,boston,141,1925884600,2764661750,
2001-08-10,18:30:57,997486257,boston,141,1929925399,2766849525,
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print