Assuming your data file looks like this:

Chunk Processor
1       1
1       2
1       2
1       3
1       0
1       2
f       1
f       2
f       4
f       4
s       6
s       3
s       2
s       1
f       3
g       2
g       6
[download]

... this might be a suitable solution. (Assuming that all data for a particular chunk is grouped together in your data file, and assuming chunk values never repeat...)

Update: If your data file is different to above, please reply or update your original post with further details - and please format your post using the writeup tips link toolic provided.

use strict;
use warnings;

my $chunk_old;
my @processor_nums;

while (my $line = <DATA>){
    my ($chunk, $processor_num) = $line =~ m{(\w*\d*)\s+(\d+)};
    
    if (!defined $chunk_old){$chunk_old = $chunk}; 

    if ($chunk eq $chunk_old){
        push @processor_nums, $processor_num;   #still on the same chu
+nk, append processor number to list.
    }
    else {
        my $result = do_calculation(\@processor_nums);  #we've got all
+ processor numbers for this chunk, now calculate.
        print "$result\n";                          #or do something e
+lse with it...
        @processor_nums = ();                   #empty the list, as we
+'re starting with the new chunk.
        push @processor_nums, $processor_num;   #save the processor nu
+mber we've read but not yet used in calculation
        $chunk_old = $chunk;                    #update the chunk flag
+ to the new chunk value 
    }
}

sub do_calculation {
    my $processor_list_ref = shift;
    my $total = 0;
    
    for my $value(@$processor_list_ref){
        #do calculation stuff here....
        $total = $total + $value;
    }
    return $total; #whatever result(s) you get
}

__DATA__
1       1
1       2
1       2
1       3
1       0
1       2
f       1
f       2
f       4
f       4
s       6
s       3
s       2
s       1
f       3
g       2
g       6
[download]