In that case, your script would print out the results for each time interval and then go on to evaluate the next lines of the logfiles, like this:

while(<>) {
        $timestring = $_ =~ $some_regexp;
        $min = &get_nr_of_mins ($timestring);
        if(/start:/) {
                /split/ and ++$split;
                /filter/ and ++$filter;
        }
        elsif(/finish:/) {
                /split/ and --$split;
                /filter/ and --$filter;
        }
        if($min % $granularity == 0) {
                print "$time: $split, $filter\n";
        }
}
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(note, that this gives you the number of processes at the time of the last line eaten from the logfile, not an average value for the last interval).

OTOH, if you want to do more sophisticated analysis of the logfile, this approach might be too simple.