From what you look to be asking, is for a good control structure to push your data through a calculation subroutine?? The best way is to is to use a subroutine.

# Init vars
$prev_xists = "0";
@output = "";
@lines = get_lines();
@outlines = calc_lines(@lines)

sub get_lines {
     ...... #perl code for getting lines
}

sub calc_lines {
     for (@_) {

#Split out your data from the line
my ($date, $time, $epochSec, $dev, $ifIdx, $ifIn, $ifOut, $nul) = spli
+t ',';
if ($prev_xists) {
my $dTime = $old_epochSec - $epochSec;
my $bps = ($ifIn + $ifOut) / $dTime;
} else {
my $bps = "";
}
$old_epochSec = $epochSec;
$prev_xists = "1";
my $new_line = "$date,$time,$dev,$ifIdx,$bps";
push @output, $new_line ;
}
return @output;
}
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This is not the most elegant solution, and there's some things I would change here and there, but this should get you started.

After all that you should have your data in @outlines.
~Hammy