calculate within a loop

[Nathalie Conte]

Hello, 
I have a question about making a calculation within a loop

I have a hash of hashes
##############
#!/usr/bin/perl
use strict;
use warnings;
 
use Data::Dumper qw(Dumper);
 
my %grades;
$grades{"Foo "}{1}   = 97;
$grades{"Foo "}{2}    = 107;
$grades{"Peti "}{1}   = 88;
$grades{"Peti "}{3}  = 89;
$grades{"Peti "}{4}          = 99;
 
print Dumper \%grades;
print "----------------\n";
 
foreach my $name (  keys %grades) {
    foreach my $subject (sort {$a <=> $b} keys %{ $grades{$name} }) {
        print "$name, $subject: $grades{$name}{$subject}\n";
    }
}

##############
output is 
$VAR1 = {
          'Peti ' => {
                       '4' => 99,
                       '1' => 88,
                       '3' => 89
                     },
          'Foo ' => {
                      '1' => 97,
                      '2' => 107
                    }
        };
----------------
Peti , 1: 88
Peti , 3: 89
Peti , 4: 99
Foo , 1: 97
Foo , 2: 107
###############
Now, what I would like to achieve:
I want to make a calculation, in each $name (Peti and Foo), calculate:
for Peti:
first line : no action
second line -(minus) 1st line:
print subject 3-1=2, 89-88=1
third line - 2nd line:
print subject 4-3=1, 99-89=10

for foo:
first line : no action
second line -(minus) 1st line:
print subject 2-1=2, 107-97=10


Many thanks for any advice
Best
Nathalie