One way is to make current inventory a hash of arrays. Assuming you get inventory on a filehandle which returns one record at a time:

my %current_inventory;
while (<RECORDS>) {
    my @ary = split ':';
    $current_inventory{$ary[0]} = [@ary[1,2]];
}
close(RECORDS) or die $!;
# now inventory is in memory as a hash, can look things up directly
[download]

Now we assume that a NEW_RECORDS handle carries the new stock. This looks almost the same:

while (<NEW_RECORDS>) {
    my @ary = split ':';
    $current_inventory{$ary[0]}->[0] += ary[1];
    $current_inventory{$ary[0]}->[1] += $ary[2]];
}
close(NEW_RECORDS) or die $!;
# now update is done, save with proper file locking
[download]

I'm not sure if the last line in the update loop matches the business logic you need.

This approach will be faster than nested loops. It requires keeping the inventory in memory, but it would be a remarkably varied inventory to make that a problem.

After Compline,
Zaxo