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

##</code><code>##

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